Phase 1-3 + 6: pluggable filter system, app factory, test harness

Implements the parallel.md workstreams (Agents A-E) toward REFACTOR_GOAL.md.

Phase 1 — app factory + blueprints + /api/v1:
- app.py -> create_app() factory (no module-level app); entrypoints updated
- routes/ (auth, pages, settings, admin, assets) + blueprints/api.py at /api/v1
- config.py / extensions.py / security.py extracted; services/ layer added
- endpoint names preserved so template url_for() calls keep resolving
  (static check: all 27 template url_for endpoints are defined routes)

Phase 2 — one pluggable filter system:
- filter_pipeline/registry.py: @register_stage / @register_plugin + discover_modules
- engine._init_stages() instantiates registered stages (no hardcoded dict);
  process_batch is AI-aware: only short-circuits to the AI-disabled path when
  a filterset's stages declare requires_ai, so offline filtersets run with AI off
- BaseFilterPlugin gets a consumer (stages/plugins.py); Keyword/Quality
  re-enabled via filter_config.json plugins config
- comment tree modes ported to stages/comment_filter.py + shared rules.py;
  wired into /api/v1/posts/<uuid> and /api/v1/comments/<uuid> via
  FilterEngine.filter_comments() (fails open)
- offline quality_filter filterset exercises plugins+ranker without AI
- legacy filter_lib / comment_lib / html_generation_lib / generate_html /
  active_html path deleted

Phase 3 prep — pluggable fetchers + Postgres models:
- Post / Comment SQLAlchemy models added to models.py
- migrate_content_to_db.py backfill (idempotent by uuid, batched, --dry-run)
- platforms/ fetcher registry (extension point)
- live reads/writes still go through PostService (disk JSON); cutover deferred

Phase 6 — test harness:
- pytest.ini + tests/ (conftest with in-memory SQLite fixture, no Postgres;
  stubbed polling/filter singletons)
- test_app_factory.py (route registration, no module-level app),
  test_api_contracts.py (posts/post_detail/comments/filters shape with
  monkeypatched post_service + get_filter_engine),
  test_filter_pipeline.py + test_plugin_contract.py (Flask-free; validated
  locally 12/12 incl. drop-in stage/plugin discovered with zero core edits)

Other: .gitignore added (__pycache__, data/, secrets); pytest in requirements.

Verification: py_compile clean across the project; the Flask-free filter-pipeline
and plugin-contract tests pass locally. App-factory / API-contract tests need
deps+docker to run; runtime flask routes / Auth0-repeated-create_app also gated
on docker.

Co-Authored-By: Claude <noreply@anthropic.com>

filter_pipeline/registry.py

@@ -0,0 +1,62 @@
+"""Registries for filter pipeline stages and plugins."""
+
+import importlib
+import logging
+from typing import Any, Dict, Iterable, Optional, Type
+
+logger = logging.getLogger(__name__)
+
+_STAGE_REGISTRY: Dict[str, Type[Any]] = {}
+_PLUGIN_REGISTRY: Dict[str, Type[Any]] = {}
+_DISCOVERED_MODULES = set()
+
+
+def register_stage(name: str):
+    """Register a pipeline stage class under a config name."""
+    def decorator(stage_cls: Type[Any]):
+        if name in _STAGE_REGISTRY and _STAGE_REGISTRY[name] is not stage_cls:
+            logger.warning("Replacing registered filter stage '%s'", name)
+        _STAGE_REGISTRY[name] = stage_cls
+        return stage_cls
+
+    return decorator
+
+
+def register_plugin(name: str):
+    """Register a filter plugin class under a config name."""
+    def decorator(plugin_cls: Type[Any]):
+        if name in _PLUGIN_REGISTRY and _PLUGIN_REGISTRY[name] is not plugin_cls:
+            logger.warning("Replacing registered filter plugin '%s'", name)
+        _PLUGIN_REGISTRY[name] = plugin_cls
+        return plugin_cls
+
+    return decorator
+
+
+def get_stage_class(name: str) -> Optional[Type[Any]]:
+    """Return a registered stage class by name."""
+    return _STAGE_REGISTRY.get(name)
+
+
+def get_plugin_class(name: str) -> Optional[Type[Any]]:
+    """Return a registered plugin class by name."""
+    return _PLUGIN_REGISTRY.get(name)
+
+
+def get_registered_stages() -> Dict[str, Type[Any]]:
+    """Return a copy of registered stage classes."""
+    return dict(_STAGE_REGISTRY)
+
+
+def get_registered_plugins() -> Dict[str, Type[Any]]:
+    """Return a copy of registered plugin classes."""
+    return dict(_PLUGIN_REGISTRY)
+
+
+def discover_modules(module_names: Iterable[str]):
+    """Import modules for registration side effects once."""
+    for module_name in module_names:
+        if not module_name or module_name in _DISCOVERED_MODULES:
+            continue
+        importlib.import_module(module_name)
+        _DISCOVERED_MODULES.add(module_name)