# P6 task 04: extract `v1fetch`'s pipeline into `fetchTarget`

**Branch:** `p6` (`git switch p6`; `git status --short` must be empty, otherwise stop)
**Commit subject:** `api: v1fetch's per-target pipeline becomes fetchTarget (no behaviour change)`

> This is the one task in P6 that edits working code. It is a **move, not a rewrite**: the
> reviewer will diff the old `v1fetch` body against `fetchTarget` line by line.

## Goal

`v1fetch` today inlines the whole per-target pipeline: fetch → recover → PDF render or clean →
question limit → digest → context → log line → envelope. The batch worker (task 05) needs the
same pipeline for one target. Move it into one method that both call:

```go
// internal/api/target.go
func (s *Server) fetchTarget(ctx context.Context, c *auth.Caller, t batch.Target) (batch.Result, error)
```

`v1fetch` keeps its exact responses. **Every existing api test must pass unchanged.**

## Files

- Copy: `_files/api/fetchtarget_test.go` → `internal/api/fetchtarget_test.go`
- Create: `internal/api/target.go` (`fetchTarget`)
- Modify: `internal/api/api.go` (`v1fetch` shrinks; `doFetch` takes a `context.Context` in place of
  the `*http.Request`, and its call sites pass `r.Context()`), `docs/implementer-log.md`

## What already exists

```go
// internal/batch (task 02)
type Options struct{ Mode, Question, Pages, Grep, JQ string; Context *bool; Recover bool }
type Target struct{ URL string; Options Options }
type Result struct{ Content string; Meta map[string]any }
```

`fetch.Error{Status int; Msg, Hint string; RetryAfter int}` is the error type `writeFetchErr`
already turns into a response.

## Rules

1. **What stays in `v1fetch`:** `s.reqN.Add(1)`, the caller check, decoding `v1Req`, and the
   `400 invalid url` for a bad body or an empty URL. Then it builds a `batch.Target` from the
   request (`URL`, and `Mode`, `Question`, `Pages`, `Grep`, `JQ`, `Context`, `Recover` into
   `Options`), calls `s.fetchTarget(r.Context(), c, t)`, and:
   - on an error, `s.writeFetchErr(w, c, req.URL, err)`;
   - on success, `writeJSON(w, 200, env)` where `env` is `res.Meta` plus `"content": res.Content`.
2. **What moves into `fetchTarget`, in the same order and unchanged:** the question truncation
   to 500 runes, the mode default and check, `wantCtx`, `doFetch`, the recover path
   (`recover.Try(ctx, …)`), the PDF branch (render, `pages`/`grep` errors, the question-chunk
   limit), the clean/raw branch, the digest, the context search, the `fetch` log line, and the
   envelope.
3. **Errors that `v1fetch` wrote directly with `writeErr` become `*fetch.Error` values with the same
   status, message and hint:** `400 invalid url` for a bad mode, `400 raw mode does not apply to
   PDFs`. `writeFetchErr` then writes them exactly as before. The only visible difference is its
   `fetch failed` log line, which is fine.
4. **The envelope:** `fetchTarget` returns `Result{Content: content, Meta: m}` where `m` holds every
   other key of today's `out` map (`mode`, `untrusted`, `source`, `warnings`, `sizes`, `timings`,
   and `pdf`, `digest`, `context` when present). Never put `content` in `Meta`.
5. **`doFetch(ctx, c, rawURL, jq, onlyMain)`:** `r.Context()` becomes `ctx`. That is the only change
   in `doFetch`. `scrape` (the other caller) passes `r.Context()`.
6. No new behaviour, no new log fields, no renamed JSON keys. If something cannot move without
   changing behaviour, stop and report it.

## Steps

- [ ] **1.** `git switch p6`; copy the given test. **2. See it fail** (`fetchTarget` undefined).
- [ ] **3.** `doFetch(ctx, …)` and its call sites. **4.** `target.go`. **5.** Shrink `v1fetch`.
- [ ] **6.** `gofmt -w`; `go test -race -count=3 ./internal/api/` → `ok`, every existing test
  included. **7.** `make gate`.
- [ ] **8.** Row `p6/04-fetchtarget`; commit.

```sh
git add internal/api docs/implementer-log.md
git commit -F .state/commit-msg.txt
```

## Done when

- The two given tests and **every existing api test** pass under `-race -count=3`, with no
  existing test file modified; `make gate` ok; the given file is byte-identical.

## Stop and report if

- An existing api test fails and fixing it would need a change to that test.