Refine incremental sync audit observability

- Add per-cycle correlation ID ([cyc:xxxxxxxx]) threaded through Python
  logs and SQL audit tables for end-to-end traceability of any divergence.
- New ProductionDataBaseSync.SyncApplyRunLog table + @CycleID on usp_SyncApply
  for per-table apply auditing (pending/merged/deleted/applied, dead/error
  counts, duration). Audit write isolated in its own TRY/CATCH outside txn.
- Capture/cleanup phases now emit full detail: Insert->Delete downgrade
  warning with record_id/log_id, dedup_skipped as apply-stall signal,
  per-file summary, and access log-id ranges on cleanup.
- Queue health check surfaces error/dead rows with recent samples instead of
  silent accumulation (previously the top cause of data divergence).
- sql_writer uses INSERT...SELECT...WHERE NOT EXISTS for observable dedup;
  idle cycles lowered to DEBUG with periodic heartbeat.
- Backward compatible: old proc callers still work (CycleID nullable; legacy
  coarse-grained logging with one-time notice).

Excluded from this commit: CODE.md, build_code_doc.py (doc generation).

sql/04_sync_apply_runlog.sql

@@ -0,0 +1,75 @@
+-- SyncApplyRunLog: per-invocation, per-target-table audit of what usp_SyncApply
+-- actually did. This closes the biggest observability gap in the pipeline: the
+-- apply phase used to be a black box ("apply done") -- queue rows could flip
+-- to 'error' or 'dead' with no trace in the service log, and there was no
+-- record of how many rows a MERGE/DELETE touched at any point in time. With
+-- this table, "what did apply do to table X around time T, and did it fail?"
+-- is a single indexed query, and CycleID joins each row back to the exact
+-- [cyc:xxxxxxxx] lines in the Python service log.
+--
+-- Written by usp_SyncApply (sql/02_sync_apply.sql) as a best-effort insert per
+-- (invocation, target table). Idle cycles write nothing (the proc's cursor
+-- only visits tables that have pending rows), so growth tracks real change
+-- traffic, not poll frequency. Retention: unmanaged by default; if it ever
+-- grows large, purge by StartedAt, e.g.
+--   DELETE FROM ProductionDataBaseSync.SyncApplyRunLog
+--    WHERE StartedAt < DATEADD(day, -90, SYSDATETIME());
+--
+-- Lives under the ProductionDataBaseSync schema (run 00_schema.sql first).
+-- Idempotent: safe to re-run. Deploy alongside the updated 02_sync_apply.sql;
+-- ordering is forgiving either way (the proc checks for this table and skips
+-- the audit insert when it is absent).
+
+IF OBJECT_ID('ProductionDataBaseSync.SyncApplyRunLog', 'U') IS NULL
+BEGIN
+    CREATE TABLE ProductionDataBaseSync.SyncApplyRunLog (
+        RunLogID        bigint IDENTITY(1,1) NOT NULL,
+        CycleID         nvarchar(40)   NULL,      -- correlation id from the Python service ([cyc:...])
+        TargetSchema    nvarchar(128)  NOT NULL,
+        TargetTable     nvarchar(255)  NOT NULL,
+        PendingCount    int            NOT NULL,  -- pending queue rows seen for this table
+        DistinctRecords int            NULL,      -- distinct RecordIDs among them
+        MergedCount     int            NULL,      -- rows affected by the MERGE (insert + update)
+        DeletedCount    int            NULL,      -- rows affected by the DELETE
+        AppliedCount    int            NULL,      -- queue rows flipped to 'applied'
+        ErrorCount      int            NULL,      -- queue rows flipped to 'error' (will retry)
+        DeadCount       int            NULL,      -- queue rows flipped to 'dead' (retries exhausted)
+        Outcome         varchar(10)    NOT NULL,  -- 'ok' | 'error'
+        ErrorMsg        nvarchar(max)  NULL,
+        StartedAt       datetime2      NOT NULL
+                        CONSTRAINT DF_SyncApplyRunLog_Started DEFAULT sysdatetime(),
+        DurationMs      int            NULL,
+        CONSTRAINT PK_SyncApplyRunLog PRIMARY KEY CLUSTERED (RunLogID)
+    );
+END
+GO
+
+-- Join back to the Python service log of one cycle.
+IF NOT EXISTS (SELECT 1 FROM sys.indexes
+               WHERE name = 'IX_SyncApplyRunLog_Cycle'
+                 AND object_id = OBJECT_ID('ProductionDataBaseSync.SyncApplyRunLog'))
+BEGIN
+    CREATE INDEX IX_SyncApplyRunLog_Cycle
+        ON ProductionDataBaseSync.SyncApplyRunLog(CycleID);
+END
+GO
+
+-- "What happened to this table around time T?"
+IF NOT EXISTS (SELECT 1 FROM sys.indexes
+               WHERE name = 'IX_SyncApplyRunLog_Table'
+                 AND object_id = OBJECT_ID('ProductionDataBaseSync.SyncApplyRunLog'))
+BEGIN
+    CREATE INDEX IX_SyncApplyRunLog_Table
+        ON ProductionDataBaseSync.SyncApplyRunLog(TargetSchema, TargetTable, StartedAt);
+END
+GO
+
+-- Fast scan for failed applies.
+IF NOT EXISTS (SELECT 1 FROM sys.indexes
+               WHERE name = 'IX_SyncApplyRunLog_Outcome'
+                 AND object_id = OBJECT_ID('ProductionDataBaseSync.SyncApplyRunLog'))
+BEGIN
+    CREATE INDEX IX_SyncApplyRunLog_Outcome
+        ON ProductionDataBaseSync.SyncApplyRunLog(Outcome, StartedAt);
+END
+GO