fix(safety): skip libraries with empty watch history instead of treating all items as unwatched

[rfsbraz]

Fixes #287

Stacked on #297 (which stacks on #283) - merge in that order.

• An empty Tautulli activity set made find_watched_data return None for every item, bypassing last_watched_threshold and the watch_status: unwatched check - one degraded Tautulli response (DB rebuild, section ID mismatch, empty auth result) could mass-delete recently watched content, up to max_actions_per_run per library
• New guard: when a library has items, uses watch-based rules in the dangerous direction (unwatched, last_watched_threshold, apply_last_watch_threshold_to_collections), and Tautulli returns zero activity, the library is skipped for the run with a loud error via the existing fail-safe handler
• watch_status: watched needs no guard (empty data already fails safe: nothing matches, nothing is deleted); libraries without watch rules are unaffected
• Genuinely never-watched libraries can opt out with allow_empty_watch_history: true (added to the schema, so the generated configuration reference picks it up); the opt-out logs a warning each run
• Tests cover both trigger rules, all four pass-through cases, the opt-out, and the deleterr-level skip (library counted failed, zero deletions)

The "activity far below previous run" degradation heuristic from #287 needs state plumbing into MediaCleaner and has real false-positive scenarios (deliberate Tautulli history cleanup), so this PR covers the empty case; partial-history hardening ties into #289's pagination guard.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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