feat: harden global exception handlers — prevent traceback leakage and unify error responses

[Arijit429]

Closes #82
Closes #145
Closes #311
Closes #394
Closes #295
Closes #374

Summary

Hardens the global exception handling layer to prevent internal stack traces
from leaking to API clients, and unifies all error responses into a consistent
JSON envelope.

Problem

The current handlers.py only catches AppError. Three other error categories
are completely unhandled:

• RequestValidationError — malformed request bodies return raw Pydantic
  error arrays with internal field paths
• HTTPException — templates.py routes use HTTPException which returns
  a different shape than forms.py which uses AppError
• Unhandled Exception — if Controller() crashes, the full Python stack
  trace including file paths and line numbers is returned to the client

This is a security risk (OWASP: Security Misconfiguration).

Changes

api/errors/handlers.py

• Added HTTPException handler for consistent error shape
• Added RequestValidationError handler with human-readable messages
• Added catch-all Exception handler — logs server-side, returns generic message
• All handlers return {"success": false, "error": {"code": "...", "message": "..."}}

api/errors/base.py

• Added missing super().__init__(message) for proper exception chaining

api/routes/forms.py

• Fixed duplicate get_template() call (was querying database twice per request)
• Wrapped Controller.fill_form() in try/except for crash protection

api/main.py

• Ensured register_exception_handlers(app) is called at startup

Testing

Verified all four error paths return uniform JSON:

• 422 — validation errors (malformed body)
• 404 — application errors (template not found)
• 400 — HTTP errors (non-PDF upload)
• 500 — unhandled errors (generic message, no stack trace)

Existing test suite passes without modification.

Real-world impact

Prevents internal stack traces from leaking to the frontend — important for
CAL FIRE deployment where error logs may be reviewed by non-technical staff.

[Arijit429]

Context

The existing handlers.py only caught AppError — three other error types (HTTPException, RequestValidationError, unhandled Exception) were leaking raw internals to the client. This PR adds handlers for all four types with a uniform response shape matching the ErrorResponse schema in common.py.

Also fixed a duplicate get_template() call in forms.py that was hitting the database twice per request.

[Arijit429]

Tested locally