fix(import): decouple scan from HTTP connection, prevent duplicate imports

- Add scan_manager: background asyncio task + Redis event store so scans
  survive UI navigation; SSE stream reads from Redis and is reconnectable
- Replace SSE-tied scan endpoint with POST /nc-scan/start + GET /nc-scan/stream
- Fix frontend: AbortController + useEffect cleanup cancels stream on unmount
  without stopping the server-side scan
- Add unique constraint on audio_versions.nc_file_path (migration 0009) to
  prevent duplicate imports from concurrent scans; handle IntegrityError
  gracefully in nc_scan with rollback + skip
- Fix API health check: use plain python instead of uv (not in dev image)
- Optimize Taskfile: fix duplicate dev:restart, add dev:fresh/dev:rebuild/
  dev:status, migrate uses run --rm, check includes typecheck

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/alembic/versions/0009_audio_version_nc_path_unique.py

@@ -0,0 +1,36 @@
+"""Add unique constraint on audio_versions.nc_file_path.
+
+Prevents duplicate imports when concurrent scans race on the same file.
+
+Revision ID: 0009_av_nc_path_uq
+Revises: 0008_drop_nc_columns
+Create Date: 2026-04-12
+"""
+
+from alembic import op
+
+revision = "0009_av_nc_path_uq"
+down_revision = "0008_drop_nc_columns"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Remove any existing duplicates first (keep the oldest version per path)
+    op.execute("""
+        DELETE FROM audio_versions
+        WHERE id NOT IN (
+            SELECT DISTINCT ON (nc_file_path) id
+            FROM audio_versions
+            ORDER BY nc_file_path, uploaded_at ASC
+        )
+    """)
+    op.create_unique_constraint(
+        "uq_audio_version_nc_file_path",
+        "audio_versions",
+        ["nc_file_path"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint("uq_audio_version_nc_file_path", "audio_versions", type_="unique")