dcache: batch job and target tables with a sliding lease, sweep and byte budget

Add the batch job store to internal/dcache (storage only; no HTTP, no worker).
New internal/dcache/batch.go holds the BatchJob/BatchTarget types and all
accessors; Open gains the batch_job/batch_target tables and indices, and DB
gains BatchH/BatchM counters.

Sliding lease: writes (UpdateBatchTarget, SetBatchJobStatus, TouchBatchJob,
MarkInFlightFailed) bump the job lease to now; reads never write. BatchJobByIDem
returns the newest (caller, idem_key) job with lease+ttl>now and status
queued/running/done/partial. UpdateBatchTarget counts targets_done exactly once
per target entering done/failed and adds len(result) to nbytes. SweepBatch
removes by lease; EvictBatchOldest evicts whole non-live jobs oldest-lease-first;
MarkInFlightFailed reconciles in-flight jobs on restart. Batch nbytes are kept out
of Size() and the content evict union.

Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>

internal/dcache/dcache.go

@@ -47,6 +47,8 @@ type DB struct {
 	SearchM   uint64
 	PDFH      uint64
 	PDFM      uint64
+	BatchH    uint64
+	BatchM    uint64
 	Evictions uint64
 }
 
@@ -110,6 +112,32 @@ func Open(path string, maxBytes int64, log *slog.Logger) (*DB, error) {
 			text TEXT,
 			PRIMARY KEY (k, page)
 		);
+		CREATE TABLE IF NOT EXISTS batch_job (
+			id            TEXT PRIMARY KEY,
+			caller        TEXT NOT NULL,
+			caller_class  TEXT NOT NULL,
+			idem_key      TEXT NOT NULL,
+			status        TEXT NOT NULL,
+			error         TEXT NOT NULL DEFAULT '',
+			targets_total INTEGER NOT NULL,
+			targets_done  INTEGER NOT NULL DEFAULT 0,
+			created_at    INTEGER NOT NULL,
+			lease         INTEGER NOT NULL,
+			nbytes        INTEGER NOT NULL DEFAULT 0
+		);
+		CREATE TABLE IF NOT EXISTS batch_target (
+			job_id  TEXT NOT NULL,
+			idx     INTEGER NOT NULL,
+			url     TEXT NOT NULL,
+			options TEXT NOT NULL,
+			status  TEXT NOT NULL,
+			result  TEXT,
+			error   TEXT,
+			hint    TEXT,
+			PRIMARY KEY (job_id, idx)
+		);
+		CREATE INDEX IF NOT EXISTS batch_job_idem ON batch_job(caller, idem_key);
+		CREATE INDEX IF NOT EXISTS batch_job_lease ON batch_job(lease);
 	`); err != nil {
 		_ = sq.Close()
 		return nil, err