fix(backend): relax CORS regex and add debugging logs

[KnellBalm]

This PR addresses a CORS issue reported in production where the frontend origin was being blocked.

Changes:

1. Regex Update: The cloud_origin_regex in backend/main.py has been updated to r"https://query-craft-frontend.*\.run\.app/?" to allow for an optional trailing slash. This ensures that even if the browser or a proxy appends a slash to the Origin header (an edge case), it will still be matched correctly.
2. Debug Logging: Added logic to log_errors_middleware to log the Origin header for requests from *.run.app domains. This will provide visibility into exactly what Origin header is reaching the backend in production, facilitating faster debugging if the issue persists.
3. Test Coverage: Added a new test case test_cors_origin_with_trailing_slash to tests/test_cors_config.py to verify the regex fix. Existing tests were also run and passed.

This change is safe as it only relaxes the regex slightly to cover valid variations of the frontend URL and adds harmless logging.

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PR created automatically by Jules for task 7501668948331363069 started by @KnellBalm

Summary by Sourcery

Relax CORS origin matching and add CORS-focused diagnostics for Cloud Run traffic.

Bug Fixes:

• Allow Cloud Run frontend origins with an optional trailing slash to pass CORS validation.

Enhancements:

• Log incoming Origin headers for run.app requests to aid CORS debugging in production.

Documentation:

• Update the implementation plan document to describe the CORS issue resolution steps instead of the previous incident.
• Document the addition of CORS regex changes, Origin logging, and test coverage in the implementation plan.

Tests:

• Add a CORS test to verify that origins with a trailing slash are accepted by the regex and keep disallowed-origin coverage intact.

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[sourcery-ai]

Reviewer's Guide

Relaxes the Cloud Run CORS origin regex to accept an optional trailing slash, adds targeted Origin-header logging for run.app traffic to aid CORS debugging, and extends CORS test coverage and documentation to cover the new behavior.

Sequence diagram for CORS request handling with new Origin logging

sequenceDiagram
    actor Browser
    participant Frontend_run_app
    participant CloudRun_Ingress
    participant Backend_FastAPI
    participant log_errors_middleware
    participant CORS_Middleware
    participant App_Handlers

    Browser->>Frontend_run_app: Fetch API request with Origin https://query-craft-frontendXYZ.run.app/
    Frontend_run_app->>CloudRun_Ingress: Forward request
    CloudRun_Ingress->>Backend_FastAPI: HTTP request with Origin header

    Backend_FastAPI->>log_errors_middleware: Process incoming request
    log_errors_middleware->>log_errors_middleware: Read Origin header
    alt Origin header contains run.app
        log_errors_middleware->>Logging_backend_middleware_cors_debug: info Incoming Request Origin: {origin}
    else Origin header missing_or_non_run_app
        log_errors_middleware->>log_errors_middleware: Skip Origin logging
    end

    log_errors_middleware->>CORS_Middleware: Call next middleware

    alt Origin matches cloud_origin_regex https://query-craft-frontend.*\.run\.app/?
        CORS_Middleware->>App_Handlers: Allow request and forward to route handler
        App_Handlers-->>CORS_Middleware: Response
        CORS_Middleware-->>Browser: Response with CORS headers
    else Origin does not match regex
        CORS_Middleware-->>Browser: CORS error response
    end

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Flow diagram for Cloud Run frontend to backend CORS and logging path

flowchart LR
    Browser[Browser] --> Frontend_run_app[Frontend Cloud Run service]
    Frontend_run_app --> CloudRun_Ingress[Cloud Run ingress]
    CloudRun_Ingress --> Backend_FastAPI[Backend FastAPI service]

    Backend_FastAPI --> log_errors_middleware[log_errors_middleware]
    log_errors_middleware --> CORS_Middleware[CORS middleware]

    CORS_Middleware -->|Origin_matches_cloud_origin_regex_https_query_craft_frontend_run_app_optional_slash| App_Handlers[Application route handlers]
    CORS_Middleware -->|Origin_does_not_match_regex| CORS_Error_Response[CORS error response]

    log_errors_middleware -->|Origin_header_contains_run_app| Logging_backend_middleware_cors_debug[Logger backend_middleware_cors_debug]
    log_errors_middleware -->|Origin_missing_or_not_run_app| CORS_Middleware

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File-Level Changes

Change	Details	Files
Relax CORS Cloud Run origin regex to allow origins with an optional trailing slash.	Update cloud_origin_regex to include an optional trailing slash using '/?' in the pattern. Ensure production CORS configuration continues to use the updated regex for Cloud Run origins.	backend/main.py
Add CORS-focused Origin header debug logging for Cloud Run traffic.	Extend log_errors_middleware to read the Origin header from incoming requests. Create a dedicated logger (backend.middleware.cors_debug) and log Origin values only when they contain 'run.app' to reduce noise.	backend/main.py
Tighten and extend CORS test coverage and docs to cover trailing-slash origins and updated plan.	Add test_cors_origin_with_trailing_slash to verify that an origin with a trailing slash is allowed and CORS headers are set. Remove overly verbose comments in existing CORS tests while preserving behavior. Update IMPLEMENTATION_PLAN documentation to describe the CORS regex fix, logging addition, and related testing, replacing outdated incident content.	tests/test_cors_config.py docs/IMPLEMENTATION_PLAN.md

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[sourcery-ai]

Hey - I've left some high level feedback:

• The relaxed cloud_origin_regex (https://query-craft-frontend.*\.run\.app/?) would be safer and more precise if it were anchored at the end (e.g. r"https://query-craft-frontend.*\.run\.app/?$") to avoid accidentally matching origins that only contain .run.app as a substring in a longer host.
• Consider moving the get_logger("backend.middleware.cors_debug") call outside log_errors_middleware (e.g. module-level) so the logger isn’t re-fetched on every request, reducing per-request overhead.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The relaxed `cloud_origin_regex` (`https://query-craft-frontend.*\.run\.app/?`) would be safer and more precise if it were anchored at the end (e.g. `r"https://query-craft-frontend.*\.run\.app/?$"`) to avoid accidentally matching origins that only contain `.run.app` as a substring in a longer host.
- Consider moving the `get_logger("backend.middleware.cors_debug")` call outside `log_errors_middleware` (e.g. module-level) so the logger isn’t re-fetched on every request, reducing per-request overhead.

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