feat(raft): server-side 307 redirect on ErrNotLeader

[osvaldoandrade]

Summary

The M2 multi-raft bench (#588) showed throughput parity-at-best vs single-shard, dominated by HTTP-level retry on `ErrNotLeader`. This PR closes that gap for the realistic case: writes that hit a follower now respond with HTTP 307 + `Location: <leader URL + original path>`. Go's `http.Client` follows 307 transparently (RFC 7231 preserves method + body) so naïve clients land on the leader in one extra round-trip instead of bouncing back with an error.

What landed

Layered to avoid circular deps:

• `pkg/domain/errors.go` (new): `LeaderHint` interface — `error + LeaderHTTPAddr() string`. Shared vocabulary between the storage layer (where the error is constructed) and the HTTP layer (where it's interpreted).
• `internal/repository/pebble.NotLeaderError`: typed error carrying `LeaderURL`. Satisfies `domain.LeaderHint`. `errors.Is(err, ErrNotLeader)` still matches via a `.Is` method on the type — existing callers don't change.
• `pebble.Replicator` interface gains `LeaderHTTPAddr() string` — `raft.DB` already exposes it (PR feat(raft): leader HTTP URL in /raft/status #589). `pebble.Set/Delete/CommitBatch` build `NotLeaderError{LeaderURL: d.repl.LeaderHTTPAddr()}` when the local node isn't the leader.
• `internal/controllers/respond.go`: `respondWriteError` + `maybeRedirectLeader` helpers. `errors.As` against `domain.LeaderHint`, write 307 + `Location` from leader URL + the original request URI.
• Updated controllers: `create_task`, `claim_task`, `submit_result`, `heartbeat`, `nack`, `abandon`. Batch endpoints kept on the per-item error path (mixed-shard batches can't redirect cleanly — a follow-up could split by shard).

Behavior on the wire

```
POST /v1/codeq/tasks → 307 Temporary Redirect
Location: http://leader-host:8080/v1/codeq/tasks
Body: {"error":"not leader","leader":"http://leader-host:8080\"}
```

A standard Go `http.Client` follows automatically and the second POST lands on the leader returning 202.

Test plan

• `TestRaft_307RedirectsToLeader` — 3-node single-shard cluster. Identifies leader + follower via probe writes; sends to follower with redirect-following disabled, asserts 307 + valid `Location`. Then re-sends with standard `http.Client` and asserts the 202. ~210 ms.
• All existing raft + pebble + app tests still pass
• Manual against the new compose template once `codeq-service:cluster` image is built — verify a curl loop with `--location` succeeds against any of the 3 nodes

🤖 Generated with Claude Code