feat(security): rate-limit public share endpoint + security headers

[DavidsonGomes]

Summary

• Vault §2.S1 CRITICAL: add flask-limiter (in-memory, single-process) — 60 req/min/IP on /api/shares/<token>/view
• Vault §2.S2: inject Referrer-Policy: no-referrer, Cache-Control: no-store, Pragma: no-cache, HSTS, X-Content-Type-Options on every public share response
• Add flask-limiter>=3.5 to pyproject.toml dependencies
• Create dashboard/backend/rate_limit.py singleton (breaks circular-import between app.py → blueprints)
• Global default 600 req/min on all routes; sensitive endpoints override with @limiter.limit()

Test plan

• 71 existing backend tests pass (excluding pre-existing test_cache_hit_does_not_call_compute_again failure unrelated to this PR)
• Manual: 61st request in 1 min to /api/shares/<token>/view → HTTP 429 + Retry-After header
• Manual: share response includes all 5 security headers

🤖 Generated with Claude Code

Summary by Sourcery

Introduce request rate limiting and stricter security headers for public share endpoints.

New Features:

• Add rate limiting to the public file share view endpoint to cap requests per IP address.
• Introduce a shared Flask-Limiter instance with a default global rate limit for all routes.

Enhancements:

• Apply standard security headers to all public share responses to improve caching, referrer, content-type, and transport security.
• Add flask-limiter as a new backend dependency.

[sourcery-ai]

Reviewer's Guide

Adds Flask-Limiter-based rate limiting and security headers to the public share view endpoint, introduces a shared limiter singleton, and wires the new dependency into the project configuration.

Sequence diagram for rate-limited public share view endpoint

sequenceDiagram
    actor Client
    participant FlaskApp
    participant Limiter
    participant SharesBlueprint
    participant AfterThisRequestHandler

    Client->>FlaskApp: GET /api/shares/<token>/view
    FlaskApp->>Limiter: check_rate_limit(remote_address, 60 per minute)
    alt over_limit
        Limiter-->>FlaskApp: limit_exceeded
        FlaskApp-->>Client: 429 Too Many Requests + Retry-After
    else within_limit
        Limiter-->>FlaskApp: allowed
        FlaskApp->>SharesBlueprint: call view_share(token)
        SharesBlueprint->>AfterThisRequestHandler: register _add_security_headers
        SharesBlueprint-->>FlaskApp: Response(file_content)
        FlaskApp->>AfterThisRequestHandler: apply _add_security_headers(Response)
        AfterThisRequestHandler-->>FlaskApp: Response with security headers
        FlaskApp-->>Client: 200 OK + file + security headers
    end

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Class diagram for limiter singleton and shares endpoint integration

classDiagram
    class Limiter {
        +list default_limits
        +string storage_uri
        +Limiter callable key_func
        +init_app(app)
        +limit(limit_string)
    }

    class rate_limit {
        +Limiter limiter
    }

    class SharesBlueprint {
        +view_share(token)
    }

    rate_limit --> Limiter : creates_instance_of
    SharesBlueprint --> rate_limit : imports_limiter
    SharesBlueprint ..> Limiter : uses_limit_decorator_on_view_share

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File-Level Changes

Change	Details	Files
Introduce rate limiting and security headers for the unauthenticated public share view endpoint.	Import after_this_request and shared limiter into the shares blueprint module Apply a 60-requests-per-minute limit decorator to the /api/shares//view route Register an after_this_request handler inside view_share to inject Referrer-Policy, Cache-Control, Pragma, X-Content-Type-Options, and Strict-Transport-Security headers on every response from the endpoint	dashboard/backend/routes/shares.py
Add flask-limiter as a backend dependency.	Declare flask-limiter>=3.5 in the project dependencies list so it is installed with the backend	pyproject.toml
Create a shared, uninitialized Flask-Limiter singleton with global defaults for use across blueprints.	Define a Limiter instance configured with get_remote_address, a default global rate of 600 requests per minute, and in-memory storage Document that app.py is responsible for calling limiter.init_app(app) and that blueprints should import limiter from this module to avoid circular imports	dashboard/backend/rate_limit.py

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[sourcery-ai]

Hey - I've left some high level feedback:

• The @after_this_request handler is defined after several early return paths in view_share, so 404/410 and other error responses from this endpoint will not receive the new security headers; consider registering the handler immediately after entering the view to cover all responses.
• The import from rate_limit import limiter may be fragile depending on how the backend package is laid out; using a relative import such as from .rate_limit import limiter can avoid issues when the app is imported as a package.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `@after_this_request` handler is defined after several early `return` paths in `view_share`, so 404/410 and other error responses from this endpoint will not receive the new security headers; consider registering the handler immediately after entering the view to cover all responses.
- The import `from rate_limit import limiter` may be fragile depending on how the backend package is laid out; using a relative import such as `from .rate_limit import limiter` can avoid issues when the app is imported as a package.

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