TASK-032: Implement geo_cache retention policy and cleanup

Add automatic cleanup of stale geolocation cache entries to prevent
unbounded database growth. Resolves the issue where unique IP addresses
accumulated indefinitely in the geo_cache table, degrading query performance.

## Changes

### Database Schema (Migration 3)
- Add 'last_seen' column to geo_cache table tracking last reference time
- Existing entries default to current timestamp

### Repository Layer (geo_cache_repo.py)
- Update upsert_entry() to set/refresh last_seen on insert/update
- Update upsert_neg_entry() to set/refresh last_seen on negative cache hits
- Update bulk_upsert_entries() to set/refresh last_seen in batch operations
- Add delete_stale_entries(db, cutoff_iso) -> int for purging old entries

### Background Task (geo_cache_cleanup.py)
- New APScheduler task that runs nightly (24-hour interval)
- Calculates cutoff as 90 days ago from current time (UTC)
- Deletes all entries with last_seen older than cutoff
- Logs operation results (info when deleted > 0, debug when 0 deleted)
- Configurable retention period via GEO_CACHE_RETENTION_DAYS constant

### Application Startup (startup.py)
- Register geo_cache_cleanup task in scheduler during app startup
- Placed after geo_cache_flush in task registration order

### Tests
- Add delete_stale_entries test cases covering:
  * Removal of old entries beyond cutoff
  * No deletion when all entries are recent
  * Empty table edge case
- Update existing test fixtures to include last_seen column
- Add full test suite for cleanup task registration and execution

### Documentation
- Architekture.md: Document cleanup task, update schema/diagram
- Backend-Development.md: Add retention policy documentation

## Behavior

When an IP is accessed, its last_seen is refreshed. After 90 days of no
access, an IP is purged by the nightly cleanup. On next encounter, the IP
is re-resolved from MaxMind MMDB or ip-api.com (if configured).

This is acceptable because:
1. Stale geolocation data may become inaccurate over time
2. Re-resolution cost is minimal compared to unbounded storage growth
3. Active IPs maintain fresh data through their last_seen updates

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/Backend-Development.md

@@ -979,6 +979,16 @@ BANGUI_GEOIP_ALLOW_HTTP_FALLBACK="true"                    # Enable HTTP fallbac
 - Failed lookups are cached for 5 minutes to avoid hammering external APIs.
 - The background `geo_cache_flush` task (runs every 60 seconds) persists newly resolved entries to the database.
 - The background `geo_re_resolve` task (configurable schedule) periodically re-resolves stale entries to keep data fresh.
+- The background `geo_cache_cleanup` task (runs nightly) removes entries not referenced in the configured retention period (default: 90 days) to prevent unbounded database growth and maintain query performance.
+
+**Retention & Cleanup:**
+
+The `geo_cache` table tracks the last time each IP was referenced via a `last_seen` timestamp. Over time, as unique IPs accumulate, the table can grow very large, degrading query performance on every geo lookup. To manage this:
+
+- The `geo_cache_cleanup` background task runs once per day (default: midnight UTC).
+- It removes all entries where `last_seen` is older than the configured retention period (default: 90 days).
+- If a purged IP is encountered again after cleanup, it will be re-resolved from the MaxMind database or ip-api.com (if configured).
+- The retention period is controlled by the constant `GEO_CACHE_RETENTION_DAYS` in `backend/app/tasks/geo_cache_cleanup.py`.
 
 ### API Documentation Configuration