Made each source start from its newest stored release, with --since as an override

Without --since, fetcher service and job-search serve start each source
from the newest release stored for it (the new store.LatestReleaseDate,
by Source), so a restart carries on where it left off instead of fetching
the last day again; with nothing stored, the client's default still
applies. --since overrides it for every source. The Find a Tender source's
name is now a constant, as the Source its opportunities are stored with.

The README's guide to adding a source is brought up to date with the
current worker, start and config type names.

README.md

@@ -84,6 +84,10 @@ refuses to open a database that isn't at this build's schema version, so run
 
 `service` logs to `logging.file` if it's set, or to the terminal with
 `--foreground`. Both commands take `--since "YYYY-MM-DD[ hh:mm:ss]"`.
+Without it, `service` (and `job-search serve`) starts each source from the
+newest release stored for it, so a restart carries on where it left off; with
+nothing stored, Find a Tender's client fetches the last day. `--since`
+overrides that for every source.
 `fts-once` can be limited with `--planning`, `--opportunities` or
 `--awards`, and with `--ignore-rate-limit` it fails on a rate limit instead
 of waiting.
@@ -169,7 +173,7 @@ Using a source called `example` as an illustration:
 1. **Configuration section:** create `internal/config/conf_source_example.go`,
    copying an existing source such as `conf_source_ftsmock.go`:
    - Add a `JSCKSourceExample = "example"` constant.
-   - Define `JobSearchConfigSourceExample`, embedding `JobSearchConfigSource`
+   - Define `JSCSourceExample`, embedding `JSCSourcesSource`
      (for `enabled` and `interval`), plus any fields of its own with `cfg` and
      optional `default` tags, e.g. ``APIRoot string `cfg:"api_root"` ``.
    - Implement the `Section` interface (see `internal/config/section.go`):
@@ -178,8 +182,8 @@ Using a source called `example` as an illustration:
      `// ---------- Section methods (see Section) ----------` banner, renaming
      the receiver type and using the new constant in `Flatten`.
 2. **Register the section:** add
-   ``Example JobSearchConfigSourceExample `cfg:"example"` `` to
-   `JobSearchConfigSources` in `internal/config/conf_sources.go`. The `cfg` tag
+   ``Example JSCSourceExample `cfg:"example"` `` to
+   `JSCSources` in `internal/config/conf_sources.go`. The `cfg` tag
    must match the constant. The source's keys (`sources.example.*`) are then
    loaded, validated and available to the `config` commands with no further
    code.
@@ -189,18 +193,27 @@ Using a source called `example` as an illustration:
    for `fts_mock`: create it with the source's own API root.
 4. **Worker:** create `internal/fetcher/worker_example.go`, following
    `worker_ftsmock.go`:
-   - `runExample(ctx, conf, since)` builds a `util.Worker` with the source's
-     name, `conf.Sources.Example.Interval` and a task, then calls `Run`.
-   - The task fetches everything since its `since` argument. Wrap blocking
-     calls in `util.AwaitTask` so shutdown isn't held up, and return
-     `ErrNoNewReleases` when nothing is found, so the checkpoint still moves
-     forward. Any other error makes the next run retry from the same point.
+   - Add an `exampleSource = "Example"` constant: the worker's name, and the
+     `Source` its opportunities are stored with (set it on each one if the
+     client doesn't, as `fetchFTSMock` does).
+   - `runExample(ctx, logger, conf, save, since)` builds a `util.Worker`
+     with `exampleSource`, `conf.Interval` and a task, then calls `Run`.
+   - The task fetches everything since its `since` argument, then returns
+     `save(ctx, logger, opportunities)`, which stores them when the fetcher
+     has a store. Wrap blocking calls in `util.AwaitTask` so shutdown isn't
+     held up, and return `ErrNoNewReleases` when nothing is found, so the
+     checkpoint still moves forward. Any other error makes the next run
+     retry from the same point.
 5. **Start it:** in `Run` in `internal/fetcher/fetcher.go`, add
    ```go
-   if conf.Sources.Example.Enabled {
-       start("Example", runExample)
+   if f.conf.Example.Enabled {
+       start(exampleSource, func(ctx context.Context, since time.Time) error {
+           return runExample(ctx, logger, &f.conf.Example, f.save, since)
+       })
    }
    ```
+   `start` passes the worker its starting point: `--since`, or the newest
+   release stored with `exampleSource`.
 6. **Tests:** `TestSections` checks the new section implements `Section`
    and that its keys match its `cfg` tag, with no changes needed. In
    `internal/config/conf_sections_test.go`, add the section to