Fix redirect correctness and analytics tracking

[AlexKitipov]

Motivation

• Make short-link redirects deterministic and ensure analytics are recorded only for valid, non-expired, active links.
• Prevent client/browser caching from skipping analytics by avoiding permanent 301 redirects.
• Persist and index expires_at so expiration checks are reliable during redirect handling.

Description

• Split redirect logic into focused helpers: _get_redirectable_link, _enforce_click_limit, and _record_click in app/api/v1/redirect.py.
• Enforce expires_at and is_active before recording analytics and enforce free-tier click limits prior to writes.
• Switch redirect responses from 301 to a temporary 302 via REDIRECT_STATUS_CODE to avoid cache bypassing analytics.
• Persist expires_at when creating links in app/api/v1/links.py and add an index on expires_at in app/models/link.py.
• Add tests/test_redirect.py with coverage for analytics insertion, temporary redirect status, free click limit blocking, inactive links, and expired links.

Testing

• Ran pytest -q and all tests passed.
• Ran flake8 on modified files and there were no lint failures.
• Verified git diff --check produced no whitespace/errors related issues.

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Add the missing Alembic migration for the new index

This model-only index does not get applied to existing Alembic-managed databases: I checked migrations/versions and the only links.expires_at definition is in 0001_initial_schema.py, which creates the column without an ix_links_expires_at index, and there is no later migration for it. In deployments that use alembic upgrade, the database schema will drift from the model and expiration-related queries/maintenance will still lack the intended index; please add a migration that creates/drops the links.expires_at index.

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