Wait on a busy SQLite database and turn on WAL mode (closes #198)

Requests and the eviction pass write on separate connections, and with
no busy timeout a write that met another one failed at once with
"database is locked" and was lost. internal/database now adds
_pragma=busy_timeout(5000) to every db_url, the default or one the
operator sets, so such a write waits up to five seconds. The default
db_url's _journal_mode=WAL is not a parameter the driver reads, so it
is now _pragma=journal_mode(WAL). README.md and config.example.yml say
what pixa adds to db_url.

Model: opus-5-5

config.example.yml

@@ -34,9 +34,10 @@ maintenance_mode: false
 state_dir: ./data
 
 # SQLite database URL (default:
-# file:<state_dir>/state.sqlite3?_journal_mode=WAL). An empty value aborts
-# startup; leave the key out to use the default.
-# db_url: "file:./data/state.sqlite3?_journal_mode=WAL"
+# file:<state_dir>/state.sqlite3?_pragma=journal_mode(WAL)). pixa adds
+# _pragma=busy_timeout(5000) to it. An empty value aborts startup; leave the
+# key out to use the default.
+# db_url: "file:./data/state.sqlite3?_pragma=journal_mode(WAL)"
 
 # Image proxy settings
 # HMAC signing key for URL signatures (required, at least 32 characters)