Refactor filter configuration with regex validation

- Add regex validation utility for query strings
- Update filter_config_service to use regex validation
- Add comprehensive test coverage for regex validator
- Update exception handling for validation errors
- Update documentation for tasks

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

Docs/Tasks.md

@@ -1,39 +1,3 @@
-## [MEDIUM] No CORS configuration
-
-**Where found**
-
-- `backend/app/main.py` — no CORS middleware added
-
-**Why this is needed**
-
-If frontend on different origin, cross-origin requests blocked without CORS configuration.
-
-**Goal**
-
-Add CORS middleware with proper origin whitelisting.
-
-**What to do**
-
-1. Add CORS middleware with specific origin whitelist
-2. Make configurable via environment variable
-3. Default to localhost for development
-
-**Possible traps and issues**
-
-- `allow_origins=["*"]` defeats CORS security
-- Credentials require specific origins, not wildcard
-- Missing config silently fails in browser
-
-**Docs changes needed**
-
-- Update `Docs/Deployment.md` § CORS Configuration
-
-**Doc references**
-
-- `Docs/Deployment.md`
-
----
-
 ## [MEDIUM] Input validation missing for regex patterns (ReDoS)
 
 **Where found**