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>

backend/app/repositories/geo_cache_repo.py

@@ -98,7 +98,8 @@ async def upsert_entry(
             country_name = excluded.country_name,
             asn          = excluded.asn,
             org          = excluded.org,
-            cached_at    = strftime('%Y-%m-%dT%H:%M:%fZ', 'now')
+            cached_at    = strftime('%Y-%m-%dT%H:%M:%fZ', 'now'),
+            last_seen    = strftime('%Y-%m-%dT%H:%M:%fZ', 'now')
         """,
         (ip, country_code, country_name, asn, org),
     )
@@ -120,7 +121,11 @@ async def upsert_entry_and_commit(
 async def upsert_neg_entry(db: aiosqlite.Connection, ip: str) -> None:
     """Record a failed lookup attempt as a negative entry."""
     await db.execute(
-        "INSERT OR IGNORE INTO geo_cache (ip) VALUES (?)",
+        """
+        INSERT INTO geo_cache (ip) VALUES (?)
+        ON CONFLICT(ip) DO UPDATE SET
+            last_seen = strftime('%Y-%m-%dT%H:%M:%fZ', 'now')
+        """,
         (ip,),
     )
 
@@ -148,7 +153,8 @@ async def bulk_upsert_entries(
             country_name = excluded.country_name,
             asn          = excluded.asn,
             org          = excluded.org,
-            cached_at    = strftime('%Y-%m-%dT%H:%M:%fZ', 'now')
+            cached_at    = strftime('%Y-%m-%dT%H:%M:%fZ', 'now'),
+            last_seen    = strftime('%Y-%m-%dT%H:%M:%fZ', 'now')
         """,
         rows,
     )
@@ -202,3 +208,21 @@ async def bulk_upsert_entries_and_neg_entries_and_commit(
         await db.commit()
 
     return positive_count, negative_count
+
+
+async def delete_stale_entries(db: aiosqlite.Connection, cutoff_iso: str) -> int:
+    """Delete geo cache entries not referenced since the cutoff timestamp.
+
+    Args:
+        db: Open BanGUI application database connection.
+        cutoff_iso: ISO 8601 timestamp (e.g., '2024-01-01T00:00:00Z'). Entries with
+            ``last_seen`` before this time will be deleted.
+
+    Returns:
+        The number of rows deleted.
+    """
+    async with db.execute(
+        "DELETE FROM geo_cache WHERE last_seen < ?",
+        (cutoff_iso,),
+    ) as cur:
+        return cur.rowcount if cur.rowcount is not None else 0