implement timestamp correction utilities and tests for git commit data

[shlokgilda]

Description

Fixes #3472 - PostgreSQL insertion failures due to invalid committer timestamps.

The Bug:
facade_bulk_insert_commits() only validated cmt_author_timestamp when handling DB insertion failures. If the author timestamp was valid but the committer timestamp had an invalid timezone (e.g., -13068837 from git corruption), the insert would fail permanently.

The Fix:

• Created augur/tasks/git/correction.py with timezone validation functions (simple string operations)
• Modified lib.py to validate both author and committer timestamps when the binary search isolates a problematic commit
• Uses author timestamp as fallback for invalid committer timestamp to minimize data loss
• Only validates on insertion failure (keeps the existing performance optimization intact)

Changes:

1. NEW: augur/tasks/git/correction.py - Timestamp validation utilities
2. MODIFIED: augur/application/db/lib.py - Now validates both timestamps on failure
3. NEW: tests/test_tasks/test_git/test_correction.py - 12 unit tests (all passing)

Notes for Reviewers

• The fix maintains the existing binary search optimization - we only validate timestamps when PostgreSQL rejects an insert, not proactively
• POSTGRES_VALID_TIMEZONES includes all real-world timezone offsets (-12:00 to +14:00). Some of the timezone offsets were missing. Reference: https://docs.oracle.com/cd/E19563-01/819-4437/anovd/index.html
• Fallback chain: invalid committer → use author timestamp → UTC (minimizes data loss per issue discussion)

Signed commits

• Yes, I signed my commits.

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AI Disclosure: I used Claude Code to assist with writing docstrings, code review, writing test cases, and drafting this PR description. All test cases were manually verified and are passing locally.