WS0 cleanup: DuckDB-free + wire DATABASE_URL

- Drop duckdb dependency (pyproject, requirements) -> psycopg2-binary.
- run_live_paper: boot log shows the real Postgres target (masked); add
  _db_target_desc() helper.
- .env.example: document DATABASE_URL / PG* vars; mark WATCHER_EVENT_DB_PATH legacy.
- evaluation/storage.py: INSERT OR REPLACE -> INSERT (setup_id is a fresh uuid).

Verified: 67/67 tests green on Postgres with duckdb UNINSTALLED; app boots
against the live endpoint (127.0.0.1:5432 proxy) and wrote 205 daily bars.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -2,6 +2,16 @@
 # Momentum — environment configuration (copy to .env and fill)
 # ============================================================
 
+# ---- Database (PostgreSQL) — the single datastore (event store + bar warehouse)
+# Either a full URL:
+DATABASE_URL=postgresql://USER:PASSWORD@HOST:5432/momentum
+# ...or individual vars (used only if DATABASE_URL is unset):
+# PGHOST=127.0.0.1
+# PGPORT=5432
+# PGUSER=admin
+# PGPASSWORD=
+# PGDATABASE=momentum
+
 # ---- Alpaca (market data + paper broker) — REQUIRED for live paper trading
 ALPACA_API_KEY=
 ALPACA_SECRET_KEY=
@@ -47,7 +57,7 @@ TRADING_MAX_DAILY_LOSS_PCT=0.03 # daily-loss circuit breaker
 # Intraday watcher (signal engine)
 WATCHER_ENABLED=1
 WATCHER_INTERVAL_SECONDS=60
-WATCHER_EVENT_DB_PATH=./data/momentum.duckdb
+# WATCHER_EVENT_DB_PATH is legacy and ignored — the datastore is PostgreSQL (see DATABASE_URL above)
 WATCHER_MAX_SYMBOLS=25
 WATCHER_MIN_BARS=30
 WATCHER_MIN_QUALITY=0.30