Optimise geo lookup and aggregation for 10k+ IPs

- Add persistent geo_cache SQLite table (db.py)
- Rewrite geo_service: batch API (100 IPs/call), two-tier cache,
  no caching of failed lookups so they are retried
- Pre-warm geo cache from DB on startup (main.py lifespan)
- Rewrite bans_by_country: SQL GROUP BY ip aggregation + lookup_batch
  instead of 2000-row fetch + asyncio.gather individual calls
- Pre-warm geo cache after blocklist import (blocklist_service)
- Add 300ms debounce to useMapData hook to cancel stale requests
- Add perf benchmark asserting <2s for 10k bans
- Add seed_10k_bans.py script for manual perf testing

backend/app/db.py

@@ -64,6 +64,17 @@ CREATE TABLE IF NOT EXISTS import_log (
 );
 """
 
+_CREATE_GEO_CACHE: str = """
+CREATE TABLE IF NOT EXISTS geo_cache (
+    ip              TEXT    PRIMARY KEY,
+    country_code    TEXT,
+    country_name    TEXT,
+    asn             TEXT,
+    org             TEXT,
+    cached_at       TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
+);
+"""
+
 # Ordered list of DDL statements to execute on initialisation.
 _SCHEMA_STATEMENTS: list[str] = [
     _CREATE_SETTINGS,
@@ -71,6 +82,7 @@ _SCHEMA_STATEMENTS: list[str] = [
     _CREATE_SESSIONS_TOKEN_INDEX,
     _CREATE_BLOCKLIST_SOURCES,
     _CREATE_IMPORT_LOG,
+    _CREATE_GEO_CACHE,
 ]