Add portable snapshot endpoint

[terjekv]

Summary

• add a synchronous, versioned portable snapshot endpoint with archive and single-file JSON representations
• export domain data from a consistent PostgreSQL transaction, with optional legacy permissions and deterministic checksums
• preserve wildcard HINFO, LOC, and SSHFP records that cannot be imported automatically as explicit deferred records
• split oversized TXT values into valid DNS character strings and tighten HTTP content negotiation and digest headers
• centralize snapshot authorization and refactor DNS translation, manifest construction, archive writing, hashing, and temporary-file cleanup into focused components

Impact

Operators can create implementation-independent backups for transfer and recovery workflows without holding a database transaction open during download. Existing users, tokens, audit history, and authorization memberships remain excluded. Snapshot access is available to members of the dedicated configured group, MREG administrators, and MREG superusers.

The JSON representation delivers a pure, single-file, dependency-ordered JSON import document, while records that cannot be imported automatically are surfaced separately for manual handling.

Validation

• complete Django suite with PostgreSQL connection pooling enabled: 999 tests, 997 passed, 2 skipped
• local fresh line coverage: 97.885%
• Coveralls coverage: 97.859%, up 0.09 percentage points from master at 97.767%
• snapshot module line coverage: 98.390%
• uv run ruff check mreg/api/v1/tests/test_snapshot.py
• uv run ruff format --check mreg/api/v1/tests/test_snapshot.py
• git diff --check

[coveralls]

coverage: 97.818% (+0.05%) from 97.765% — feature/portable-snapshot into master