refactor: Remove duplicate router-level exception helpers

All routers now let domain exceptions propagate to the global handlers in main.py
instead of catching and converting them to HTTPException. This eliminates:

- Duplicate exception-to-HTTP-status mappings across 8 routers
- Duplicate helper functions (_bad_gateway, _not_found, _conflict, etc.)
- Inconsistent error response formats

Changes:
- Removed all try/except blocks from routers that catch domain exceptions
- Removed duplicate helper functions from all routers
- Added missing exception handlers to main.py for:
  * ActionNameError
  * FilterNameError
  * JailNameError
  * JailNotFoundInConfigError
  * FilterInvalidRegexError
- Removed unused imports from affected routers

All domain exceptions now propagate to the single authoritative mapping in
main.py, ensuring consistent error codes, messages, and logging across the API.

Affected routers:
- action_config.py: Removed _action_not_found, _bad_request, _not_found helpers
- bans.py: Removed try/except in ban/unban endpoints
- config_misc.py: Removed try/except blocks
- file_config.py: Removed 6 try/except blocks and _service_unavailable helper
- filter_config.py: Removed try/except blocks
- geo.py: Removed try/except in lookup_ip endpoint
- jail_config.py: Removed try/except blocks
- jails.py: Removed try/except blocks
- server.py: Removed try/except blocks

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

Docs/Backend-Development.md

@@ -251,6 +251,33 @@ async def jail_not_found_handler(request: Request, exc: JailNotFoundError) -> JS
     return JSONResponse(status_code=404, content={"detail": f"Jail '{exc.name}' not found"})
 ```
 
+### Routers and Exception Propagation
+
+- **Routers must NOT construct `HTTPException` for domain errors** — let domain exceptions propagate.
+- Routers should never have helper functions like `_bad_gateway()`, `_not_found()`, `_conflict()` etc. that convert domain exceptions to `HTTPException`.
+- All domain exception types must have corresponding handlers registered in `main.py` via `app.add_exception_handler()`.
+- Exception handlers are registered in order from most specific to least specific — FastAPI evaluates them in registration order.
+
+```python
+# ❌ BAD — routers constructing HTTPException for domain exceptions
+@router.get("/{name}")
+async def get_jail(name: str, socket_path: Fail2BanSocketDep) -> JailDetailResponse:
+    try:
+        return await jail_service.get_jail(socket_path, name)
+    except JailNotFoundError:
+        raise HTTPException(status_code=404, detail=f"Jail not found: {name!r}") from None
+
+# ✅ GOOD — domain exception propagates to global handler
+@router.get("/{name}")
+async def get_jail(name: str, socket_path: Fail2BanSocketDep) -> JailDetailResponse:
+    return await jail_service.get_jail(socket_path, name)
+```
+
+All domain exceptions raised by services propagate to handlers in `main.py`, ensuring:
+1. Consistent error response format across the entire API.
+2. No duplicated exception-to-HTTP-status mapping logic.
+3. Easy to audit all error codes — they are all in one place.
+
 ---
 
 ## 9. Testing