T-04: Encapsulate geo_service module-level mutable state in GeoCache class

Create GeoCache class with all mutable state as instance attributes:
- _cache, _neg_cache, _dirty, _geoip_reader, _geoip_initialized, _cache_lock
- All public methods: lookup(), lookup_batch(), lookup_cached_only(), flush_dirty(), load_from_db(), clear(), etc.

Initialization & Dependency Injection:
- Instantiate GeoCache in startup.py and store on app.state.geo_cache
- Add get_geo_cache() dependency function in dependencies.py
- Inject into routes and tasks via FastAPI's dependency system

Backward Compatibility:
- Maintain module-level functions in geo_service.py as deprecated wrappers
- All old callers continue to work through _default_geo_cache instance
- Remove test-escape-hatch functions (clear_cache, clear_neg_cache moved to methods)

Background Tasks:
- Update geo_cache_flush.py and geo_re_resolve.py to receive GeoCache instance
- Tasks now operate on injected instance rather than module globals

Tests:
- Refactor test_geo_service.py with geo_cache fixture providing fresh instances
- Update patch paths to target GeoCache methods correctly
- Fix internal state assertions to access instance attributes

Documentation:
- Update Architekture.md to document GeoCache as managed stateful service
- Describe cache lifecycle (load on startup, flush periodically, re-resolve stale)
- Note process-local limitations for multi-worker deployments

Fixes violation of Single Responsibility Principle: module no longer owns both
lookup logic and cache lifecycle management. Cache is now a first-class
injectable service with transparent lifecycle.

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

Docs/Tasks.md

@@ -1,23 +1,3 @@
-### T-03 · Centralise `_DEFAULT_PAGE_SIZE` constant
-
-**Where found:** `backend/app/routers/dashboard.py:45`, `routers/history.py:34`, `services/ban_service.py:70`, `services/history_service.py:49`
-
-**Why this is needed:** Four independent definitions can drift. The router default and service default are currently coincidentally aligned at 100, but nothing enforces this.
-
-**Goal:** Single definition in `app/utils/constants.py`, imported everywhere.
-
-**What to do:**
-1. Add `DEFAULT_PAGE_SIZE: Final[int] = 100` and `MAX_PAGE_SIZE: Final[int] = 500` to `app/utils/constants.py`.
-2. Replace all four local `_DEFAULT_PAGE_SIZE` and `_MAX_PAGE_SIZE` declarations with imports.
-
-**Possible traps and issues:** None significant. Pure search-and-replace.
-
-**Docs changes needed:** None.
-
-**Doc references:** `app/utils/constants.py`
-
----
-
 ### T-04 · Encapsulate `geo_service` module-level mutable state in a class
 
 **Where found:** `backend/app/services/geo_service.py` — module globals `_cache`, `_neg_cache`, `_dirty`, `_geoip_reader`, `_geoip_initialized`, `_cache_lock`