feat(06-02): add just worker target and document worker in README

- justfile: worker target depends on db-up, passes MinIO dev defaults
  (DATABASE_URL, S3_ENDPOINT/BUCKET/REGION/ACCESS_KEY/SECRET_KEY/USE_PATH_STYLE)
- README: replace skeleton section with full "Running the Worker" docs
  (just worker command, expected logs, single-worker constraint, graceful shutdown,
   failed job retry observation)

backend/README.md

@@ -66,7 +66,7 @@ identical. If you don't have podman:
 backend/
   cmd/
     web/main.go            # HTTP server entry point
-    worker/main.go         # background worker (skeleton — boot/log/shutdown only)
+    worker/main.go         # background worker — river periodic jobs (Phase 6)
   internal/
     db/                    # pgxpool wiring + sqlc-generated queries
     web/                   # chi router, handlers, middleware, design-system
@@ -123,17 +123,61 @@ Every command in this table is a recipe in `backend/justfile`.
 | `just build`                                    | Generates assets, then builds `bin/web` and `bin/worker`                     | Producing release binaries locally                       |
 | `just clean`                                    | Removes `bin/`, `tmp/`, `static/htmx.min.js`, `static/tailwind.css`, and `*_templ.go` files | Reset to a fresh-clone state without dropping the Postgres volume |
 
-## Worker (skeleton — Phase 1 only)
+## Running the Worker
 
-`cmd/worker` in Phase 1 boots, logs `worker ready`, and idles waiting for a
-signal. Real job runtime lands in Phase 6 (D-03). To run it manually:
+`cmd/worker` is the background job processor. It runs river periodic jobs against
+the same Postgres as `cmd/web`. Start it with:
 
 ```
-DATABASE_URL=postgres://xtablo:xtablo@localhost:5432/xtablo?sslmode=disable \
-  go run ./cmd/worker
+just worker
 ```
 
-Ctrl-C to exit.
+This requires `just db-up` (handled automatically as a dependency) and MinIO
+running (used by the orphan-file cleanup job). If MinIO is not running, the worker
+will exit on startup with "file store init failed".
+
+### What to expect
+
+- Structured logs appear immediately at startup.
+- A `"worker ready"` log line appears within a few seconds after `rivermigrate`
+  and S3 init complete.
+- A `"worker heartbeat"` log line appears almost immediately (the heartbeat job
+  is configured with `RunOnStart: true`, so it fires on the first scheduler tick
+  which happens within seconds of startup).
+- Subsequent heartbeat logs appear every ~1 minute.
+- The orphan-file cleanup job runs every hour (no `RunOnStart` — first run is
+  ~1 hour after startup).
+
+### Single-worker constraint
+
+**Run only one worker process at a time (v1).** River uses advisory locks for
+leader election and concurrent rivermigrate runs are unsafe. Do not run multiple
+worker instances against the same database in this version.
+
+### Graceful shutdown
+
+Send SIGINT (Ctrl+C) and observe:
+
+```
+{"level":"INFO","msg":"shutting down"}
+{"level":"INFO","msg":"shutdown complete"}
+```
+
+The worker calls `riverClient.StopAndCancel` with a 10-second timeout, which
+cancels in-flight job contexts and waits for goroutines to exit before closing
+the pool.
+
+### Observing failed job retries
+
+River logs each failure via the `SlogErrorHandler`. A failed job produces a log
+line like:
+
+```
+{"level":"ERROR","msg":"job error","job_id":42,"job_kind":"heartbeat","attempt":1,"max_attempts":25,"err":"..."}
+```
+
+River retries up to 25 times with exponential backoff (`attempts^4` + jitter).
+After 25 failed attempts the job is moved to the discarded state in `river_job`.
 
 ## Troubleshooting
 

backend/justfile

@@ -112,6 +112,17 @@ dev: db-up
     just generate
     DATABASE_URL='{{ database_url }}' SESSION_SECRET=191affeb1624de1f0e07bd5cfab14cd655510a24f7e673bd784ea56847890caf air -c .air.toml
 
+# Start the worker binary (development — requires db-up and MinIO running).
+worker: db-up
+    DATABASE_URL='{{ database_url }}' \
+    S3_ENDPOINT='http://localhost:9000' \
+    S3_BUCKET='xtablo' \
+    S3_REGION='us-east-1' \
+    S3_ACCESS_KEY='minioadmin' \
+    S3_SECRET_KEY='minioadmin' \
+    S3_USE_PATH_STYLE='true' \
+    go run ./cmd/worker
+
 test:
     just generate
     go test ./...