perf(storage): prune error_log on startup with 90-day retention

The error_log table had no retention policy: every backend error was
appended forever, so across months of dogfooding it grew unbounded. That
silently bloats the diagnostic-bundle export and slows the
list_recent_errors query the Settings → About panel runs.

- New `kon_storage::prune_error_log(pool, keep_days)` does a single
  `DELETE FROM error_log WHERE timestamp < datetime('now', '-Nd days')`
  and returns the row count removed.
- src-tauri/src/lib.rs runs it once during setup() with a const
  ERROR_LOG_RETENTION_DAYS = 90. Failure is logged to stderr but does not
  block startup — a prune that fails is strictly less important than the
  app coming up.
- Test: insert three rows at now / -30d / -200d, verify a 90-day prune
  removes only the oldest, and a subsequent 14-day prune removes the
  -30d row. Storage suite at 60/60.

https://claude.ai/code/session_0189xUb6ie6t9qHkzatGZ9Rb

crates/storage/src/lib.rs

@@ -14,7 +14,8 @@ pub use database::{
     insert_transcript, list_feedback_examples, list_implementation_rules, list_profile_terms,
     list_profiles, list_recent_completions, list_recent_errors, list_subtasks, list_tasks,
     list_transcripts, list_transcripts_paged, log_error, mark_implementation_rule_fired,
-    record_feedback, search_transcripts, set_implementation_rule_enabled, set_setting,
+    prune_error_log, record_feedback, search_transcripts, set_implementation_rule_enabled,
+    set_setting,
     set_task_energy, uncomplete_task, update_profile, update_task, update_transcript,
     update_transcript_meta, DailyCompletionCount, ErrorLogRow, FeedbackRow, FeedbackTargetType,
     ImplementationRuleRow, InsertTranscriptParams, ProfileRow, ProfileTermRow,