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. A new test checks that this list and configs/config.example.yml
name exactly the keys of the table.

Model: opus-5-5

internal/config/config_validation_internal_test.go

@@ -15,8 +15,36 @@ import (
 	_ "modernc.org/sqlite" // SQLite driver registration
 )
 
+// The config keys the tests name in the errors they expect.
+const (
+	keyDebug                      = "debug"
+	keyMaintenanceMode            = "maintenance_mode"
+	keyPort                       = "port"
+	keyStateDir                   = "state_dir"
+	keySentryDSN                  = "sentry_dsn"
+	keyDBURL                      = "db_url"
+	keyMetrics                    = "metrics"
+	keyMetricsUsername            = "metrics.username"
+	keyMetricsPassword            = "metrics.password"
+	keySigningKey                 = "signing_key"
+	keyAllowlistHosts             = "allowlist_hosts"
+	keyRefererBlocklist           = "referer_blocklist"
+	keyAllowHTTP                  = "allow_http"
+	keyUpstreamConnectionsPerHost = "upstream_connections_per_host"
+	keyUpstreamConnections        = "upstream_connections"
+	keyMaxConcurrentProcessing    = "max_concurrent_processing"
+	keyCacheMaxBytes              = "cache_max_bytes"
+	keyBlockedNetworks            = "blocked_networks"
+	keyTrustedProxies             = "trusted_proxies"
+	keyAccessControlAllowOrigin   = "access_control_allow_origin"
+	keyUpstreamFetchTimeout       = "upstream_fetch_timeout"
+	keyUpstreamMaxResponseSize    = "upstream_max_response_size"
+	keyDownstreamTimeout          = "downstream_timeout"
+	keyImageRequestsPerMinute     = "image_requests_per_minute"
+)
+
 // validTestSigningKey is a 32-character signing key that satisfies the
-// minimum length requirement in validate().
+// minimum length requirement in validateSigningKey.
 const validTestSigningKey = "0123456789abcdef0123456789abcdef"
 
 // signingKeyLine is a valid signing_key config line used as the base of