Added CODE_CONVENTIONS.md and linters to enforce it, and applied it across the codebase

CODE_CONVENTIONS.md sets out the conventions for Go code in this
repository: naming, layout, comments, errors, logging, configuration and
tests. .golangci.json enables the linters that check the parts that can be
checked (each limited to the rules the conventions use); the rest is
applied by hand.

Applied across the codebase:
- Naming: full-word names in application code, initialisms in one case
  (BuyerID, PrintJSON, PrintYAML, ocdsGetBuyerID), no names shadowing
  built-ins (min/max are minGap/maxGap), mustRaw renamed rawJSON as it
  returns nil rather than panicking, and config.ConfigLevelToLogLevel
  renamed LevelToLogLevel.
- Layout: lines kept to 80 characters where possible, assignments taken
  out of if and for statements that don't fit, wrapped arguments all on
  one line or one per line, steps separated by blank lines, no else after
  a return, function and struct literals wrapped.
- Comments: doc comments on every exported name (including each section's
  Map, Flatten, TypeOf, ValueOf and SetValue), section banners, British
  spelling.
- Errors: a created error starts with its package name, a wrapping error
  adds only what was being done, sentinels in one commented var block,
  errors.Is and errors.As in place of == and type assertions, and no
  err := shadowing of a named result. That fixes the mock server losing
  its access log's close error.
- Logging: the fetcher passes its workers an untagged logger, so
  findatender's lines are tagged with their own component rather than
  with fetcher's as well; cmd/mock-fts logs "stopped" with slog.
- Configuration: util.WithTimeout (moved from the websocket package)
  applies a timeout of 0 or less as no limit, used for the websocket
  write and ping, TCP close and tcpserver shutdown timeouts; every setting
  documents what 0 or empty means, including the rate limit rule's keys
  in internal/config/README.md.
- Tests: related tests merged into table-driven ones with named cases.

Separately from CODE_CONVENTIONS.md, internal/store/README.md gains a soft
SQL style convention for the queries in internal/store/queries.go, and the
queries are reformatted to follow it.

internal/config/README.md

@@ -48,6 +48,9 @@ is kept.
 | `server.websocket.max_message_size` | byte size | `32KiB` | Largest message read from a client; `0B` for 32KiB. `-1` (no limit) is rejected, as a client could then exhaust the server's memory |
 | `server.websocket.connection_limit.*` | | | A rate limit rule for new connections from each client IP, WebSocket and plain TCP alike; limiters are kept for `server.rate_limits.idle_ttl` |
 | `server.websocket.message_rate_limit.*` | | | A rate limit rule for messages from each connection |
+| `server.websocket.*.enabled` | bool | `false` | In a rate limit rule: whether it applies |
+| `server.websocket.*.per_second` | int | `50` | In a rate limit rule: sustained rate. `0` or less: the rule is off |
+| `server.websocket.*.burst` | int | `100` | In a rate limit rule: largest burst. Below `1`: 1 |
 | `server.websocket.write_timeout` | duration | `10s` | `0s` for none |
 | `server.websocket.ping_interval` | duration | `30s` | How long a connection may be idle before it is pinged; `0s`: no pings |
 | `server.websocket.pong_timeout` | duration | `10s` | `0s` for none |
@@ -91,7 +94,7 @@ per key:
   passed to `SetValue` (e.g. `"10m"`, `"true"`).
 - A tagged field whose type is a struct is a nested section, addressed as
   `section.key`. An embedded struct has its fields promoted, as Go does;
-  every source embeds `JobSearchConfigSource` for `enabled` and `interval`.
+  every source embeds `JSCSourcesSource` for `enabled` and `interval`.
 - `time.Duration` fields report `reflect.String` from `TypeOf`, accept either a
   `time.Duration` or a duration string in `SetValue`, and are given out as
   strings (`"10m0s"`) by `Map`, `Flatten` and `ValueOf`, and so by `JSON`,
@@ -125,7 +128,7 @@ To add a new section:
    its key, and an implementation of `Section` (copy an existing section's
    methods).
 2. Add the struct as a tagged field of its parent: `JobSearchConfig` in
-   `config.go`, or `JobSearchConfigSources` in `conf_sources.go` for a source,
+   `config.go`, or `JSCSources` in `conf_sources.go` for a source,
    etc. The field's `cfg` tag must match the `JSCK*` constant.
 
 Nothing in `section.go` needs changing.