refactor(backend): move service-layer HTTP errors to API boundary mapping

[peteski22]

Summary

• Introduce a service-layer exception hierarchy and domain exceptions for auth, API keys, knowledge, review, and users.
• Remove fastapi.HTTPException usage from backend services and raise service/domain errors instead.
• Map service exceptions to HTTP status codes at route/app boundary, including centralized handling in cq_server.app.

Validation

• make lint
• make test

[Copilot]

Pull request overview

This PR refactors the backend to stop raising fastapi.HTTPException from service-layer code, introducing a service/domain exception hierarchy and centralizing translation of those exceptions to HTTP responses at the API boundary (FastAPI app/router layer).

Changes:

• Added a cq_server.exceptions hierarchy (auth, API keys, knowledge, review, users) and updated services to raise these errors instead of HTTPException.
• Added centralized ServiceError → HTTP status mapping via a global FastAPI exception handler in cq_server.app.
• Updated dependencies and tests to reflect the new service-layer behavior (notably API key auth and last-used tracking).

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
server/backend/tests/test_deps.py	Updates API key service tests for new authenticate() signature and touch behavior.
server/backend/tests/test_auth_service.py	Adds unit tests for AuthService.login raising service-layer exceptions.
server/backend/src/cq_server/services/reviews.py	Replaces HTTPException with review/knowledge service exceptions.
server/backend/src/cq_server/services/knowledge.py	Replaces HTTPException with knowledge service exceptions and updates docstrings.
server/backend/src/cq_server/services/auth.py	Replaces HTTPException with InvalidCredentialsError.
server/backend/src/cq_server/services/api_keys.py	Replaces HTTPException with API key/user service exceptions; splits out touch_last_used.
server/backend/src/cq_server/exceptions/base.py	Introduces base ServiceError and HTTP-category subclasses.
server/backend/src/cq_server/exceptions/auth.py	Adds auth-specific service exceptions.
server/backend/src/cq_server/exceptions/api_keys.py	Adds API key-specific service exceptions.
server/backend/src/cq_server/exceptions/knowledge.py	Adds knowledge-specific service exceptions.
server/backend/src/cq_server/exceptions/review.py	Adds review-specific service exceptions.
server/backend/src/cq_server/exceptions/users.py	Adds user-specific service exceptions.
server/backend/src/cq_server/exceptions/init.py	Exports exception types for convenient importing.
server/backend/src/cq_server/app.py	Adds a global FastAPI exception handler mapping ServiceError to HTTP responses.
server/backend/src/cq_server/api/routes/auth.py	Adds auth route exception→status mapping helper.
server/backend/src/cq_server/api/routes/knowledge.py	Adds knowledge route exception→status mapping helper.
server/backend/src/cq_server/api/routes/review.py	Adds review route exception→status mapping helper.
server/backend/src/cq_server/api/routes/users.py	Adds user route exception→status mapping helper.
server/backend/src/cq_server/api/deps.py	Moves API key last_used_at update scheduling into the dependency; converts service error to HTTPException.

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