CI performance: add deterministic backend dependency caching

[Konan69]

Context

PR #180 removes shard and artifact fan-in complexity. Dependency installation remains outside the measured semantic-lane execution time and should only be optimized after hosted timing identifies it as material.

Outcome

Add deterministic, commit-safe backend dependency caching without caching test results, databases, coverage data, or mutable runtime state.

Scope

• Cache only immutable package-manager/download artifacts.
• Key caches by operating system, architecture, Python version, package-manager version, and the complete dependency lock/config hash.
• Restore safely on cache miss or corruption.
• Keep editable project code, PostgreSQL data, MinIO data, pytest state, and coverage files uncached.
• Measure hosted cold-cache and warm-cache behavior.

Acceptance criteria

• Cache keys include every dependency-resolution input.
• A cache miss executes the same install and verification path as today.
• Cached content cannot suppress dependency installation errors.
• No credentials or environment-specific configuration enter cache keys or archives.
• Tests, databases, MinIO state, coverage, and generated evidence are explicitly excluded.
• Cold and warm hosted timings are recorded across multiple successful runs.
• The cache is removed if it does not produce a material, stable improvement.

Non-goals

• Test-result caching.
• Path-based test omission.
• Cross-commit database or coverage reuse.