fix: widen signed_at/signed_by columns to prevent PostgreSQL overflow

[bradleygauthier]

Summary

signed_at column (String(30)) was 2 characters too narrow for datetime.isoformat() output from timezone-aware datetimes (32 chars for UTC +00:00), causing StorageError on every PostgreSQL write. SQLite was unaffected (it ignores VARCHAR length). Also widens signed_by from String(16) to String(32) — the legacy 16-char hex fingerprint had zero headroom.

• Widen signed_at from String(30) to String(40) in both CapsuleModel and CapsuleModelPG
• Widen signed_by from String(16) to String(32) in both models
• Bump version to 1.5.1

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Documentation update
• Test improvement

Checklist

Python reference (reference/python/)

• Tests pass: cd reference/python && pytest tests/ — 761 passed, 7 skipped
• Linter passes: cd reference/python && ruff check src/ tests/ — All checks passed
• Type checker passes: cd reference/python && mypy src/qp_capsule/ — Success
• Golden fixtures pass: cd reference/python && pytest tests/test_golden_fixtures.py

TypeScript reference (reference/typescript/)

• Tests pass — N/A (no TypeScript changes)
• Type check passes — N/A
• Conformance passes — N/A

General

• Documentation updated — Storage Schema section added to API docs with full column reference and migration guide
• CHANGELOG.md updated — [1.5.1] - 2026-03-17 with Fixed + Migration sections

Protocol Impact

• No protocol impact (implementation only)

Migration

For existing PostgreSQL deployments:

ALTER TABLE quantumpipes_capsules
  ALTER COLUMN signed_at TYPE VARCHAR(40),
  ALTER COLUMN signed_by TYPE VARCHAR(32);

New installations get the correct widths automatically via create_all(). SQLite databases require no action.

Test Coverage

24 new regression tests in test_column_width.py:

• Schema introspection (4): verify both models declare the correct column widths
• Isoformat length validation (7): prove UTC-aware strings exceed old limit, fit new limit
• Fingerprint format lengths (2): legacy hex prefix + keyring qp_key_XXXX both fit
• SQLite roundtrip (4): signed_at/signed_by survive get, list, get_latest
• PostgreSQL roundtrip (7): same paths plus get_all_ordered, keyring format, and post-roundtrip verify()