chore: add .env.example and wire compose.yml to env vars

- .env.example documents all env vars with defaults (postgres creds, DATABASE_URL, TTLs, pool sizes)
- compose.yml reads postgres credentials and app config from .env via env_file
- .gitignore: track .env.example, ignore .env

.gitignore

@@ -2,9 +2,8 @@
 /goyomi
 /cmd/server/server
 
-# Environment
+# Environment (copy .env.example → .env and fill in values)
 .env
-.env.*
 
 # Go tooling
 *.test