Load every setting from one table in internal/config (closes #320)

Config.settings lists each setting once: its key, its environment variable
and the function that loads it. Most rows load through field, whose
arguments give the type, the default and the check. db_url and
cache_max_bytes keep their own load functions. Loading, the unknown-key
and unknown-variable checks and the names in error messages all read the
table. allowlist_hosts now loads through parseHostList.
isKnownConfigKey, envVarNames, strictLoader, validate,
validateAllowlistHostsValue and getStringSlice are gone, with the
getStringSlice tests. The key constants move to the tests, their only
users. README.md lists the settings one item each instead of in a table,
with shorter sentences above the list. A new test checks that this list
and configs/config.example.yml name exactly the keys of the table.

Model: opus-5-5

TODO.md

@@ -30,6 +30,30 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-26 one table in `internal/config` lists every setting once (closes
+  #320). `Config.settings` gives each setting's key, its environment variable
+  and the function that loads it.
+    - Most rows load through `field`, whose arguments give the setting's type,
+      its default and its check.
+    - `db_url` and `cache_max_bytes` have their own load functions. The default
+      of `db_url` comes from `state_dir`, and `cache_max_bytes` records whether
+      it is set.
+    - Loading, the unknown-key check, the unknown-variable check and the names
+      in error messages all read the table.
+    - `allowlist_hosts` loads as `referer_blocklist` does, through
+      `parseHostList`.
+    - The metrics username and password are checked together once the table has
+      loaded.
+    - When a config holds more than one invalid value, the error names the one
+      that comes first in the table.
+    - `README.md` lists the settings one item each, where it had a table. A test
+      checks that this list and `configs/config.example.yml` name exactly the
+      keys of the table.
+    - The key constants moved to the tests, their only users.
+    - Removed:
+        - `isKnownConfigKey`, `envVarNames`, `strictLoader` and `validate`
+        - `validateAllowlistHostsValue` and `getStringSlice`, with the three
+          tests of `getStringSlice`
 - 2026-10-26 a fetched image's format is detected from its bytes once, as an
   `imagerequest.Format` (closes #319). The upstream `Content-Type` check in the
   fetcher stays as the filter before downloading.