docs: document promotion audit sequencing fix and Storage transaction model

[cursor]

Docs added/updated

CHANGELOG.md

Added a Fixed entry under Unreleased for PR #5 (promotion audit sequencing):

• Storage.insert_promotion_record previously used connect() (autocommit) instead of transaction() (BEGIN IMMEDIATE), allowing concurrent writers to interleave and produce non-contiguous audit_seq values on the policy-fail path.
• Also notes the utc_now consolidation (removed duplicate in storage.py, now sourced from models).

src/flightdeck/storage.py

Three targeted docstrings added; no behavior changes:

1. Storage class docstring — explains the connect() vs transaction() split, the two promotion write paths (insert_promotion_record for policy-fail, commit_promotion for policy-pass), and the audit_seq monotonicity contract checked by flightdeck doctor.

2. transaction() context manager docstring — explains BEGIN IMMEDIATE semantics, when to use it over a bare connect(), and why it matters for audit_seq correctness.

3. insert_promotion_record() docstring — clarifies this is the policy-fail path (audit record only, no pointer update) vs commit_promotion (atomic audit + pointer update on policy-pass).

Codepaths covered

• Storage.insert_promotion_record / Storage.commit_promotion / Storage.transaction (src/flightdeck/storage.py)
• Promotion and rollback CLI paths (src/flightdeck/cli/main.py — context only)
• flightdeck doctor audit-seq check (referenced in class docstring)

Key knowledge gaps addressed

• Why blocked promotions still appear in the audit trail (and the transaction fix that makes this safe)
• The distinction between connect() (read queries, autocommit) and transaction() (all writes)
• The audit_seq monotonicity invariant and how it is maintained under concurrent access
• PR Database audit sequence, utc_now #5 was previously undocumented in CHANGELOG despite being a correctness fix to the audit ledger