Switch production config to env vars with defaults

Remove hardcoded S3 settings and dev-specific defaults from production
docker-compose, use .env instead of .env.production, and comment out
development-only values in .env.example

.env.example

@@ -10,14 +10,14 @@ APP_VERSION=2.0.0
 DEBUG=false
 SECRET_KEY=changez-moi-par-une-valeur-longue-et-aleatoire
 
-# ── Serveur ──────────────────────────────────────────────────
-HOST=0.0.0.0
-PORT=8000
+# ── Serveur (pour dev seulement) ──────────────────────────────
+# HOST=0.0.0.0
+# PORT=8000
 
 # ── Base de données ──────────────────────────────────────────
 # SQLite (dev):  sqlite+aiosqlite:///./data/imago.db
-# PostgreSQL:    postgresql+asyncpg://user:pass@host:5432/imago
-DATABASE_URL=sqlite+aiosqlite:///./data/imago.db
+PostgreSQL:    postgresql+asyncpg://user:pass@host:5432/imago
+# DATABASE_URL=sqlite+aiosqlite:///./data/imago.db
 
 # ── Stockage ─────────────────────────────────────────────────
 # local (fichiers sur disque) ou s3 (MinIO / AWS S3 / Cloudflare R2)
@@ -25,6 +25,7 @@ STORAGE_BACKEND=local
 UPLOAD_DIR=./data/uploads
 THUMBNAILS_DIR=./data/thumbnails
 MAX_UPLOAD_SIZE_MB=50
+SIGNED_URL_SECRET=changez-moi-en-prod
 
 # ── Stockage S3 (si STORAGE_BACKEND=s3) ──────────────────────
 S3_BUCKET=imago