feat(store): V008 pdf_ocr_events migration + record/prune API (Enhancement 2)

Add migrations/V008__pdf_ocr_events.sql with the events table + 3
indices (doc_id, run_id, ts). SqliteStore gains two pub fn:
record_pdf_ocr_event (insert one OCR sample) and prune_pdf_ocr_events
(delete rows older than retention_days; returns the affected row
count). Both follow the existing Mutex<Connection> lock pattern.

Wiring into ingest path lands in the next commit.

Closure r1 F2: explicit lock acquisition in both methods.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

migrations/V008__pdf_ocr_events.sql

@@ -0,0 +1,21 @@
+-- v0.20.x r2 Enhancement 2: PDF OCR events SQLite mirror.
+-- Stores per-page OCR samples for corpus-wide latency / failure analysis.
+CREATE TABLE pdf_ocr_events (
+  id               INTEGER PRIMARY KEY,
+  run_id           TEXT    NOT NULL,
+  ts               TEXT    NOT NULL,   -- ISO 8601 UTC (RFC 3339)
+  doc_id           TEXT,               -- nullable (detect-skip path)
+  doc_path         TEXT    NOT NULL,
+  page             INTEGER NOT NULL,
+  image_byte_size  INTEGER,
+  image_width      INTEGER,
+  image_height     INTEGER,
+  ms               INTEGER NOT NULL,
+  chars            INTEGER NOT NULL,
+  success          INTEGER NOT NULL,   -- 0 = fail, 1 = success
+  reason           TEXT,               -- "timeout" / "ocr_error" / NULL
+  ocr_engine       TEXT    NOT NULL
+);
+CREATE INDEX idx_pdf_ocr_events_doc_id ON pdf_ocr_events(doc_id);
+CREATE INDEX idx_pdf_ocr_events_run_id ON pdf_ocr_events(run_id);
+CREATE INDEX idx_pdf_ocr_events_ts     ON pdf_ocr_events(ts);