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>

config.py

@@ -0,0 +1,43 @@
+"""Application configuration.
+
+Centralizes environment-driven Flask config and app-wide constants so they
+can be imported by the app factory, services, and route modules without each
+reaching into ``os.getenv`` independently.
+"""
+
+import os
+
+from dotenv import load_dotenv
+
+# Load .env once when the config module is imported.
+load_dotenv()
+
+
+class Config:
+    """Flask configuration loaded from the environment."""
+
+    SECRET_KEY = os.getenv("SECRET_KEY", "dev-secret-key-change-in-production")
+    MAX_CONTENT_LENGTH = 16 * 1024 * 1024  # 16MB max request body (avatar uploads)
+    ALLOW_ANONYMOUS_ACCESS = os.getenv("ALLOW_ANONYMOUS_ACCESS", "true").lower() == "true"
+
+    # Branding
+    APP_NAME = os.getenv("APP_NAME", "BalanceBoard")
+    LOGO_PATH = os.getenv("LOGO_PATH", "logo.png")
+
+    # Auth0 (optional; empty disables OAuth login)
+    AUTH0_DOMAIN = os.getenv("AUTH0_DOMAIN", "")
+    AUTH0_CLIENT_ID = os.getenv("AUTH0_CLIENT_ID", "")
+    AUTH0_CLIENT_SECRET = os.getenv("AUTH0_CLIENT_SECRET", "")
+    AUTH0_AUDIENCE = os.getenv("AUTH0_AUDIENCE", "")
+
+
+# App-wide constants (not Flask config keys). Imported directly by modules.
+ALLOWED_EXTENSIONS = {"png", "jpg", "jpeg", "gif"}
+UPLOAD_FOLDER = "static/avatars"
+MAX_FILENAME_LENGTH = 100
+DEFAULT_PORT = 5021
+DEFAULT_PAGE_SIZE = 20
+MIN_PASSWORD_LENGTH = 8
+MAX_USERNAME_LENGTH = 80
+MAX_EMAIL_LENGTH = 120
+MAX_COMMUNITY_NAME_LENGTH = 100