feat: add SQLAlchemy declarative base, Alembic environment, and gitignore fix

Configures stable constraint naming so autogeneration produces clean diffs.
Alembic uses the sync psycopg driver while the runtime app uses asyncpg.

Also fixes a .gitignore bug caught during this task: the bare 'models/'
rule for the ML weights volume was matching backend/app/models/ (Python
package). Anchored all volume rules to repo root (/images/, /import/,
/downloads/, /models/, /postgres_data/, /redis_data/).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -42,13 +42,14 @@ yarn-error.log*
 extension/*.xpi
 extension/web-ext-artifacts/
 
-# Runtime volumes (must never be tracked)
-images/
-import/
-downloads/
-models/
-postgres_data/
-redis_data/
+# Runtime volumes (must never be tracked) — anchored to repo root so they
+# don't accidentally match Python package dirs like backend/app/models/
+/images/
+/import/
+/downloads/
+/models/
+/postgres_data/
+/redis_data/
 
 # IDE / OS
 .vscode/