Fix exception handler overlap issue - add DomainError catch-all handler

**Problem:** Broad exception handlers created fragility where adding a new
DomainError subclass without explicit registration would silently fall through
to the generic exception handler, losing the specific error_code and metadata.

**Solution:**
1. Import DomainError in main.py for explicit handler registration
2. Fix type hints in exception handlers from 'Exception' to specific types
   - NotFoundError handler now typed as 'NotFoundError'
   - BadRequestError handler now typed as 'BadRequestError'
   - ConflictError handler now typed as 'ConflictError'
   - DomainError handler now typed as 'DomainError'
   - ServiceUnavailableError handler now typed as 'ServiceUnavailableError'
3. Add DomainError as an explicit catch-all handler in the registration chain
   - Positioned after specific handlers, before HTTPException
   - Any unregistered DomainError subclass now gets correct error_code + metadata
4. Document the exception handler hierarchy with detailed comments
5. Update Backend-Development.md with handler hierarchy documentation
6. Update Architekture.md section 2.2 with exception handler details
7. Fix test expectations in test_main.py to verify ErrorResponse format

**Impact:** Any new DomainError subclass now automatically gets correct HTTP 500
status, error_code, and metadata - even if developer forgets explicit handler.

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

Docs/Tasks.md

@@ -1,62 +1,3 @@
-## [Backend] Pydantic validators execute at import time
-
-**Status:** ✅ **COMPLETE** — Codebase is fully compliant
-
-**Verification:**
-- ✓ Audited all 14 model files in `backend/app/models/`
-- ✓ No model files import from `app.config`, `app.services`, `app.utils`, or `app.routers`
-- ✓ No validators, field defaults, or computed fields call runtime-dependent functions
-- ✓ `BanGuiBaseModel` is pure with no side-effects
-- ✓ Path validation correctly placed in `app.utils.path_utils` and called from routers/services only
-
-**Current Architecture (Correct):**
-
-The codebase follows the proper pattern:
-1. **Models** (`app/models/`) contain only pure data classes and stateless validators (if any)
-2. **Validation requiring app state** (settings, file I/O, database) happens in **routers or services** at request time
-3. **Path validation helper** (`app.utils.path_utils.validate_log_path()`) is called from routers/services, never imported in models
-
-Example (correct pattern in place):
-```python
-# ✅ Correct: Validation in router with access to settings
-from app.utils.path_utils import validate_log_path
-
-@router.post("/jails/{name}/logpath")
-async def add_log_path(name: str, body: AddLogPathRequest) -> None:
-    validate_log_path(body.log_path)  # Called at request time
-    await config_service.add_log_path(name, body)
-```
-
-**Documentation Updates (Completed):**
-- ✓ `Docs/Architekture.md` § 2.1: Updated Models section with explicit constraints on I/O and side effects
-- ✓ `Docs/Backend-Development.md`: Enhanced validator documentation with import-time execution explanation
-
-**Important Note on Task Description:**
-
-The original task suggested using `@model_validator` with imports inside the method. This approach is **incorrect** and would not solve the problem:
-```python
-# ❌ WRONG — Still executes at import time:
-@model_validator(mode="after")
-def validate_range(self):
-    from app.config import get_settings  # ← Decorator still runs at class definition time
-    ...
-```
-
-The **correct solution** (already in place) is to perform validation in the **router or service layer** where settings and services are available at request/execution time.
-
-**Regression Prevention:**
-
-To prevent future violations, run this check before committing:
-```bash
-# Verify no app-layer imports in model files
-find backend/app/models -name "*.py" -exec grep -l "from app\.\(config\|services\|utils\|routers\)" {} \;
-# Should return: 0 files (empty)
-```
-
-Consider adding this as a pre-commit hook or CI check for long-term prevention.
-
----
-
 ## [Backend] Exception handler overlap — broad handlers catching everything
 
 **Where found**