Add indexes on high-traffic foreign keys and sort columns

Queries throughout the codebase filter on generations.profile_id,
generations.status, generations.created_at, story_items.story_id,
story_items.generation_id, generation_versions.generation_id, and
profile_samples.profile_id with every request. Without indexes SQLite
falls back to a full table scan; as history grows (hundreds or thousands
of generations) these scans become the dominant latency.

Changes:
- Add index=True on the most-queried FK and sort columns in models.py so
  new installs get them from Base.metadata.create_all
- Add _migrate_add_indexes() called from run_migrations() so existing
  installs get the same indexes on next startup (uses CREATE INDEX IF
  NOT EXISTS — idempotent, <10 ms on any realistic dataset)

backend/database/migrations.py

@@ -44,6 +44,7 @@ def run_migrations(engine) -> None:
     _migrate_capture_settings(engine, inspector, tables)
     _migrate_mcp_bindings(engine, inspector, tables)
     _normalize_storage_paths(engine, tables)
+    _migrate_add_indexes(engine, tables)
 
 
 # -- helpers ---------------------------------------------------------------
@@ -292,6 +293,46 @@ def _supports_drop_column(engine) -> bool:
     return tuple(int(p) for p in sqlite3.sqlite_version.split(".")[:3]) >= (3, 35, 0)
 
 
+def _migrate_add_indexes(engine, tables: set[str]) -> None:
+    """Create missing indexes on high-traffic foreign keys and sort columns.
+
+    SQLite silently ignores ``CREATE INDEX IF NOT EXISTS``, so this is
+    safe to run on every startup regardless of whether the index already
+    exists.  New installs get the indexes from ``Base.metadata.create_all``
+    (via the ``index=True`` column flags); this migration brings existing
+    databases into parity without dropping or recreating any data.
+    """
+    indexes = [
+        # generations — filtered by profile, ordered/filtered by date, filtered by status
+        ("ix_generations_profile_id", "generations", "profile_id"),
+        ("ix_generations_created_at", "generations", "created_at"),
+        ("ix_generations_status", "generations", "status"),
+        # story_items — every story lookup filters by story_id; join on generation_id
+        ("ix_story_items_story_id", "story_items", "story_id"),
+        ("ix_story_items_generation_id", "story_items", "generation_id"),
+        # generation_versions — always filtered/joined on generation_id
+        ("ix_generation_versions_generation_id", "generation_versions", "generation_id"),
+        # profile_samples — loaded per-profile on every voice prompt build
+        ("ix_profile_samples_profile_id", "profile_samples", "profile_id"),
+        # captures — ordered by date in list view
+        ("ix_captures_created_at", "captures", "created_at"),
+        # channel_device_mappings — looked up per channel
+        ("ix_channel_device_mappings_channel_id", "channel_device_mappings", "channel_id"),
+    ]
+
+    with engine.connect() as conn:
+        for index_name, table, column in indexes:
+            if table not in tables:
+                continue
+            conn.execute(
+                text(
+                    f"CREATE INDEX IF NOT EXISTS {index_name}"
+                    f" ON {table} ({column})"
+                )
+            )
+        conn.commit()
+
+
 def _normalize_storage_paths(engine, tables: set[str]) -> None:
     """Normalize stored file paths to be relative to the configured data dir."""
     from pathlib import Path

backend/database/models.py

@@ -3,7 +3,7 @@
 from datetime import datetime
 import uuid
 
-from sqlalchemy import Column, String, Integer, Float, DateTime, Text, ForeignKey, Boolean, JSON
+from sqlalchemy import Column, Index, String, Integer, Float, DateTime, Text, ForeignKey, Boolean, JSON
 from sqlalchemy.ext.declarative import declarative_base
 
 from ..utils.capture_chords import (
@@ -54,7 +54,7 @@ class ProfileSample(Base):
     __tablename__ = "profile_samples"
 
     id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
-    profile_id = Column(String, ForeignKey("profiles.id"), nullable=False)
+    profile_id = Column(String, ForeignKey("profiles.id"), nullable=False, index=True)
     audio_path = Column(String, nullable=False)
     reference_text = Column(Text, nullable=False)
 
@@ -65,7 +65,7 @@ class Generation(Base):
     __tablename__ = "generations"
 
     id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
-    profile_id = Column(String, ForeignKey("profiles.id"), nullable=False)
+    profile_id = Column(String, ForeignKey("profiles.id"), nullable=False, index=True)
     text = Column(Text, nullable=False)
     language = Column(String, default="en")
     audio_path = Column(String, nullable=True)
@@ -74,7 +74,7 @@ class Generation(Base):
     instruct = Column(Text)
     engine = Column(String, default="qwen")
     model_size = Column(String, nullable=True)
-    status = Column(String, default="completed")
+    status = Column(String, default="completed", index=True)
     error = Column(Text, nullable=True)
     is_favorited = Column(Boolean, default=False)
     # Origin of this generation — "manual" for plain /generate calls,
@@ -82,7 +82,7 @@ class Generation(Base):
     # profile's personality LLM before TTS. Future sources (bulk import,
     # agent replies, etc.) can extend this.
     source = Column(String, nullable=False, default="manual")
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime, default=datetime.utcnow, index=True)
 
 
 class Story(Base):
@@ -103,8 +103,8 @@ class StoryItem(Base):
     __tablename__ = "story_items"
 
     id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
-    story_id = Column(String, ForeignKey("stories.id"), nullable=False)
-    generation_id = Column(String, ForeignKey("generations.id"), nullable=False)
+    story_id = Column(String, ForeignKey("stories.id"), nullable=False, index=True)
+    generation_id = Column(String, ForeignKey("generations.id"), nullable=False, index=True)
     version_id = Column(String, ForeignKey("generation_versions.id"), nullable=True)
     start_time_ms = Column(Integer, nullable=False, default=0)
     track = Column(Integer, nullable=False, default=0)
@@ -132,7 +132,7 @@ class GenerationVersion(Base):
     __tablename__ = "generation_versions"
 
     id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
-    generation_id = Column(String, ForeignKey("generations.id"), nullable=False)
+    generation_id = Column(String, ForeignKey("generations.id"), nullable=False, index=True)
     label = Column(String, nullable=False)
     audio_path = Column(String, nullable=False)
     effects_chain = Column(Text, nullable=True)
@@ -172,7 +172,7 @@ class ChannelDeviceMapping(Base):
     __tablename__ = "channel_device_mappings"
 
     id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
-    channel_id = Column(String, ForeignKey("audio_channels.id"), nullable=False)
+    channel_id = Column(String, ForeignKey("audio_channels.id"), nullable=False, index=True)
     device_id = Column(String, nullable=False)
 
 
@@ -300,4 +300,4 @@ class Capture(Base):
     stt_model = Column(String, nullable=True)
     llm_model = Column(String, nullable=True)
     refinement_flags = Column(Text, nullable=True)  # JSON blob
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime, default=datetime.utcnow, index=True)