fix(dictation): make microphone lifecycle race-safe

Coalesce and invalidate microphone acquisition, preserve immediate stop/cancel events, block overlapping/finalising takes, and add the opt-in keep_mic_warm setting with privacy-preserving default off.\n\nVerified: frontend CI; keep_mic_warm migration default and idempotency; diff checks.

backend/database/migrations.py

@@ -243,6 +243,13 @@ def _migrate_capture_settings(engine, inspector, tables: set[str]) -> None:
             "hotkey_enabled BOOLEAN NOT NULL DEFAULT 0",
             "hotkey_enabled",
         )
+    if "keep_mic_warm" not in columns:
+        _add_column(
+            engine,
+            "capture_settings",
+            "keep_mic_warm BOOLEAN NOT NULL DEFAULT 0",
+            "keep_mic_warm",
+        )
 
 
 def _migrate_mcp_bindings(engine, inspector, tables: set[str]) -> None: