feat: trigger local retention pruning after upload (#71)

[lan17]

Summary

• wire LocalRetentionPruner into runtime assembly and ClipPipeline
• trigger pruning on clip ingestion (reason=clip_arrived) and once on startup (backstop)
• keep deletion eligibility strict (storage_uri present + status == DONE)
• switch retention config to max_local_size_bytes and add wiring helpers
• keep single-flight prune policy (drop while active)
• add behavioral coverage for trigger semantics, single-flight pruning, and startup backstop behavior

Validation

• uv run pytest tests/homesec/test_retention_pruner.py tests/homesec/test_pipeline.py tests/homesec/test_runtime_assembly.py tests/homesec/test_pipeline_events.py tests/homesec/test_integration.py
• make check

Ticket

• https://www.notion.so/311d8336c59f81b9abb4fd3fced64351

[codecov]

Codecov Report

❌ Patch coverage is 86.86131% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.56%. Comparing base (54b4a47) to head (230f837).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/homesec/retention/pruner.py	86.07%	11 Missing ⚠️
src/homesec/pipeline/core.py	86.48%	5 Missing ⚠️
src/homesec/runtime/assembly.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   82.40%   82.56%   +0.16%     
==========================================
  Files         102      104       +2     
  Lines        8922     9010      +88     
==========================================
+ Hits         7352     7439      +87     
- Misses       1570     1571       +1     

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