fix(security): add API key rotation and optional TTL expiry

[anshul23102]

Problem

backend/data/.api_key stores the single authentication secret as a plaintext file with no TTL, rotation endpoint, or revocation mechanism. Any process with read access to that directory (a co-located container, a path-traversal exploit, or a leaked backup) obtains a credential that is valid indefinitely.

Fix

• Key file is now JSON ({"key": ..., "created_at": <epoch seconds>}) instead of bare plaintext, so the key's age can be tracked. A pre-existing plaintext key file is migrated in place on next startup -- the key itself is kept, just wrapped with a fresh created_at.
• POST /api/v1/admin/api-key/rotate generates a new key and invalidates the old one immediately -- no grace period. Gated by the separate, statically-configured admin API key so a leaked client key can never be used to mint itself a replacement.
• GET /api/v1/admin/api-key/status reports the key's age and, if configured, its expiry, without exposing the key itself.
• New SECUSCAN_API_KEY_TTL_SECONDS setting (default 0 = disabled, so existing deployments are unaffected until an operator opts in). When set, both require_api_key and the session-creation endpoint reject the client key once it's older than the TTL, forcing rotation.
• Documented the new flow in docs/api-authentication.md.

Testing

• testing/backend/unit/test_api_key_rotation.py: 15 new tests covering the JSON key format, legacy-plaintext migration, rotate/status endpoint auth gating, immediate old-key invalidation, and TTL expiry on both the main auth dependency and session creation
• Updated test_auth.py and test_api_auth.py: existing init_api_key tests asserted the key file was bare plaintext; updated to read the new JSON format (the key value itself is unchanged)
• pytest testing/backend/unit -q -m "not benchmark" -- 2240 passed, 1 pre-existing failure unrelated to this change (parser sandbox timeout test, flaky in this environment)
• ruff check -- all checks passed

Fixes #1619

[utksh1]

The rotation/TTL idea is valuable, but there's a compatibility regression: init_api_key() now treats any JSON parse failure as legacy plaintext and immediately rewrites the file. For operators using SECUSCAN_API_KEY_FILE pointing at a read-only secret mount (Docker/K8s), the first upgraded boot will fail with a write error.

Fix: skip the in-place rewrite when the file is read-only or when a custom key-file path is set. Also update the README's cat backend/data/.api_key instruction since the file format changed to JSON.