Fix API DB connection: build URL from POSTGRES_* (auto-encode password)

The api crash-looped because DATABASE_URL had an un-encoded special-char
password (@/!), so SQLAlchemy parsed part of the password as the host
("...@db" → name resolution failure).

db.py now prefers building the connection from POSTGRES_USER/PASSWORD/DB via
SQLAlchemy URL.create(), which encodes the password automatically — any
password works with no manual escaping. DATABASE_URL remains an optional
override (still must be hand-encoded if used). docker-compose now passes the
POSTGRES_* vars to the api container; DEPLOYMENT.md updated (incl. a Portainer
env-vars note).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -19,7 +19,14 @@ services:
     build: .
     container_name: wp_api
     environment:
-      DATABASE_URL: ${DATABASE_URL}
+      # Preferred: the API builds its own connection string from these and
+      # encodes the password automatically (no manual URL-encoding needed).
+      POSTGRES_USER: ${POSTGRES_USER}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
+      POSTGRES_DB: ${POSTGRES_DB}
+      POSTGRES_HOST: db
+      # Optional full-URL override (must be URL-encoded if used).
+      DATABASE_URL: ${DATABASE_URL:-}
     restart: unless-stopped
     depends_on:
       db: