fix(ingest): remediate comprehensive review findings

[jmagar]

The ingestion snapshot review found storage-admission bypasses, retained secrets, unbounded allocations, retry/idempotency gaps, shutdown ownership bugs, and qualification drift. This change remediates all 23 consolidated findings across syslog, OTLP, file-tail, transcript, shell-history, heartbeat, deployment, documentation, and live-test paths.

Key behavior changes:

• direct agent ingest obeys storage admission; fleet file-tail records receive canonical enrichment and idempotent persistence
• raw AI frames are scrubbed, remote transcript paths cannot trigger local filesystem lookup, and transcript discovery/read windows are bounded
• OTLP log batches reserve queue capacity atomically and metrics enforce point plus normalized-byte budgets before materialization
• file-tail, shell-history, Docker, TCP, writer shutdown, and spool retry paths preserve cursor and task ownership
• live OTLP and negative-query checks validate protocol results; deployment sync and operator documentation reflect the actual runtime

Validation:

• cargo nextest run --locked --offline --no-fail-fast --test-threads 4 — 3,600 passed, 4 skipped
• cargo clippy --all-targets --locked -- -D warnings
• cargo fmt --check
• cargo check --all-targets --locked
• cargo check --locked --manifest-path tests/live/fixtures/ingest/otlpgen/Cargo.toml
• cargo xtask check-version-sync
• just validate-plugin
• shell syntax and diff hygiene checks

Tracking: unraid-mcp-hhp6