feat: logging

[RagingRedRiot]

What

Adds structured logging across the server with tracing, filling in all ~148 logging TODOs and replacing the stray println!s. This completes the server-side half of the generic-error design: the client still receives a generic event while the discarded error becomes the log payload.

Conventions

src/logging.rs is the single source of truth (levels, targets, fields, redaction, retention).

• Levels: server faults → error!; security denials & failed logins → warn!; lifecycle, admin actions & reaper summaries → info!; client-caused noise (bad JSON, undersized frames) → debug!.
• Client-input constraint violations (duplicate username/room name, empty content, malformed attachment, empty emoji) → warn!, not error!.
• Audit stream: a dedicated relay::audit target for security events — authn outcomes, authz / owner-gated denials, and admin actions (promote/demote/delete/reset, account & room creation, ownership grants, invites, server control). Routable separately, e.g. RUST_LOG=relay::audit=info.
• Correlation: a per-connection conn span carries who and, after auth, user_id, so dispatch-side events are attributable. Actors run as separate tasks and carry the relevant ids explicitly.
• Never log secrets: Password/PasswordHash already redact in Debug.

Config & output

• New Config.log_level (RUST_LOG still takes precedence) and Config.log_format (pretty | json | journald); main.rs builds the subscriber accordingly.
• Native systemd journald via tracing-journald (proper PRIORITY and indexed fields), falling back to stdout when there's no journal socket.

Notable behavior changes

• AuthResult gains an Error variant so a server-side auth fault is no longer audited as a "login failed".
• Account-creation audit is centralized in the user actor, covering both the open-signup and authenticated-creation paths.
• gate_room's Ok(None) | Err(_) arms are split so a real DB fault logs at error! while an existence-hidden "no room" stays debug!.
• reap() logs a per-run summary with per-table delete counts.

Retention

Logs go to stdout/journald, never the database, so they are not subject to the reaper; retention is a platform concern (documented in src/logging.rs).

Testing

• 174 integration tests pass, 0 failures.
• cargo clippy and cargo fmt --check clean.