api: v1fetch's per-target pipeline becomes fetchTarget (no behaviour change)

Move v1fetch's inline per-target pipeline into a new internal/api/target.go
method, `fetchTarget(ctx, c, t batch.Target) (batch.Result, error)`, so the
batch worker can drive the same work. The body moves unchanged in order:
question truncation, mode default/check, wantCtx, doFetch, the recover path,
the PDF branch (render, pages/grep errors, question-chunk limit), the
clean/raw branch, digest, context search, the fetch log line and the
envelope. `content` is returned as Result.Content; every other envelope key
goes in Result.Meta, never content. v1fetch keeps reqN/caller/decode/400,
builds a batch.Target from the request, calls fetchTarget, writeFetchErr on
error and writeJSON(w, 200, env) on success where env is a copy of Meta plus
"content". doFetch takes context.Context instead of *http.Request; scrape
passes r.Context(). No new behaviour, fields or JSON keys.

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

docs/implementer-log.md

@@ -21,6 +21,7 @@ One row per task session that ends in a commit. Newest last.
 | p6/02-runner-core | 2026-09-26 | done | 1 | gate: ok | caller-cleanup hand-off to task 03 | Runner core in new internal/batch with no workers (task 03 adds them; jobs stay queued, so a queued job counts as live for the caps). Public surface in batch.go: Options/Target/Result/TargetError, FetchFunc, Job/TargetStatus, Hooks and Config with defaults (TTL 24h, 32 targets/job, 1 job/caller, 8 total, worker clamped to 2, 64 MiB, 2 s/10 m retry). Submit runs the given order — empty→ErrNoTargets, idempotency via hex SHA-256 of `name+NUL+json.Marshal(targets)` (a live hit returns the id reused, touching no caps), then TargetsPerJob/JobsPerCaller/JobsTotal caps, then stores the job queued (random 16-byte hex id, CreatedAt=Lease=now) and its targets (pending, Options=json of t.Options) before returning so a queued job is live; the submitting caller is held in memory by job id, never persisted. Status maps rows and decodes Result without writing; Touch moves the lease; Cancel sets a queued job cancelled at once and leaves its targets pending; Close is idempotent. Store errors are Warn-logged with the job id, never a URL/host. |
 | p6/03-runner-workers | 2026-09-26 | done | 1 | gate: ok | none | Workers in new internal/batch/worker.go: one rotation of live jobs in submit order, round-robin via an absolute cursor (advances in pick, wraps at access, so it survives a second job being added while the first is in flight — the mod-n cursor from the design stuck at 0 for one job and re-picked it), serial per job (target i+1 never starts before i is stored), one target per fetch under the Close-cancelled context (panic→ErrInternal, never echoed; ErrRetryLater retried after RetryWait, bounded by RetryFor then stored failed with the wrapped text). Terminal status once per job: all done→done, all failed→failed, mix→partial. Cancel of a running job sets cancelled, stores the in-flight target without overwriting cancelled, leaves later targets pending. Close sets closed, cancels the context, broadcasts, waits the workers, idempotent. batch.go: New starts cfg.Workers, Submit appends the liveJob and wakes, Cancel handles the in-flight case, Close stops. Absolute cursor is the one deviation from the design's "index in live" comment. All race tests pass -count=3. | |
 | p6/04-metrics | 2026-09-26 | done | 1 | gate: ok | none | Batch series in new internal/metrics/batch.go, registered in New: `BatchJobsTotal`/`BatchTargetsTotal` CounterVecs (label "outcome", any value outside the known set folded to "other" via `batchJobOutcomes`/`batchTargetOutcomes` maps so a raw string never becomes a label), `BatchQueueDepth`/`BatchTargetsInflight` gauges, and `BatchJobSeconds`/`BatchTargetSeconds` histograms (durationBuckets) plus `BatchResultBytes` (1K,4K,16K,64K,256K,1M). Nil-receiver-safe `Observe…`/`Set…` methods mirroring ObservePDF/ObserveDigest. Series served through the existing /metrics handler; no new endpoint, no Snapshot change. Given test byte-identical; metrics tests pass under -race -count=3; make gate ok. |
+| p6/05-fetchtarget | 2026-09-26 | done | 1 | gate: ok | none | Extracted v1fetch's per-target pipeline into new `internal/api/target.go`: `fetchTarget(ctx, c, t batch.Target) (batch.Result, error)`. Body moved unchanged in order — question truncation to 500 runes, mode default/check, wantCtx, doFetch, recover path, PDF branch (render, pages/grep errors, question-chunk limit), clean/raw branch, digest, context search, the fetch log line, and the envelope. `content` is returned as `Result.Content`; every other envelope key (`mode`,`untrusted`,`source`,`warnings`,`sizes`,`timings`, and `pdf`/`digest`/`context` when present) goes in `Result.Meta`, never content. `v1fetch` keeps reqN/caller/decode/400, builds a `batch.Target` (`URL` + `Mode`/`Question`/`Pages`/`Grep`/`JQ`/`Context`/`Recover` into `Options`), calls `fetchTarget`, `writeFetchErr` on error and `writeJSON(w,200,env)` on success where `env` is a fresh copy of `Meta` plus `"content"` (a copy, so `len(env)==len(Meta)+1` holds and Meta is not mutated). The two `writeErr` failures become `*fetch.Error` with the same status/msg/hint: 400 "invalid url" (bad mode), 400 "raw mode does not apply to PDFs". `doFetch` takes `context.Context` instead of `*http.Request` (its three `r.Context()` → `ctx`); `scrape` passes `r.Context()`. Removed now-unused `fmt`/`unicode/utf8`/`recover` imports, added `batch`. All api tests pass under -race -count=3; make gate ok; given test byte-identical. |
 
 ## P5a review (owner, 2026-09-26)