sqlite: _busy_timeout/_journal_mode DSN params are ignored by modernc (busy_timeout=0 in server/ingestor) #576

Found in the #571 review (Minimax). modernc.org/sqlite v1.34.5 silently ignores mattn-style DSN parameters such as _journal_mode and _busy_timeout; its applyQueryParams only handles _pragma, _time_format and _txlock.

Affected call sites:

• cmd/server/db.go:211
• cmd/server/ping_score_history.go:451
• cmd/ingestor/admin_delete_client_rx.go:104

Verified: OpenDB on a WAL file reads back busy_timeout=0. WAL mode only holds because the file is already WAL.

Impact: server readers and ingestor writers get an immediate SQLITE_BUSY instead of waiting. This plausibly explains intermittent "database is locked" errors in tests and possibly in production.

Fix: switch to _pragma=busy_timeout(N) and _pragma=journal_mode(WAL), which is the modernc syntax, in every DSN. Add a test that opens each DSN and asserts the effective PRAGMA busy_timeout and journal_mode. The server must stay mode=ro (Kpa-clawbot#1283).

Relates to #571 / #567.