feat: propagate persistence sentinel and flush retries

[Teagan42]

What

• introduce a dedicated persistence sentinel across the loader pipeline and ensure retry queues flush before shutdown
• update enrichment and loader coordination to emit the new sentinel and drain pending retry batches safely
• extend test coverage for sentinel propagation, retry flushing, and queue cleanup while bumping the project version

Why

• persistence workers must shut down cleanly once upstream stages finish while guaranteeing pending work and retries are not lost

Affects

• loader pipeline coordination, persistence retry handling, and associated tests

Testing

• uv run ruff check .
• uv run pytest

Documentation

• not needed

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https://chatgpt.com/codex/tasks/task_e_68e2d93080188328832d662a86167b1f

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
mcp_plex/loader
__init__.py	746	129	83%	174–181, 230, 316–321, 923–924, 968, 970, 1035–1036, 1047–1049, 1052, 1055–1056, 1058, 1080, 1090, 1101–1133, 1148, 1150, 1157–1171, 1174–1188, 1194, 1209–1240, 1245–1278, 1281–1285, 1290–1298, 1308, 1376–1378, 1439–1441, 1799
mcp_plex/loader/pipeline
channels.py	73	2	97%	19–20
enrichment.py	334	44	87%	78, 80, 87, 91, 134–137, 170, 191, 211, 219–221, 228–231, 234–236, 244, 301, 357, 380, 384, 386, 412, 430, 465, 471, 474–482, 508, 532, 603, 606–608
ingestion.py	81	8	90%	67, 98–108, 129, 157, 179
persistence.py	117	9	92%	109, 151–152, 162, 167, 171–173, 201
mcp_plex/server
__init__.py	614	29	95%	43–44, 119–120, 148, 252, 256, 277–280, 297, 362, 365, 402, 420–421, 458, 1109, 1131–1137, 1173, 1191, 1196, 1214, 1338, 1375
__main__.py	4	4	0%	3–8
config.py	48	7	85%	50, 52–55, 65, 76
TOTAL	2227	232	90%

Tests	Skipped	Failures	Errors	Time
125	0 💤	0 ❌	0 🔥	47.098s ⏱️