Merge branch 'main' into golangci-v2.12.2

Resolves conflicts with the startup config validation from #53:
internal/config/config.go takes main's validation implementation
wholesale, with getStringSlice mechanically adapted to this branch's
keyless signature; TODO.md keeps both Completed Steps entries.

TODO.md

@@ -34,6 +34,12 @@ fill up
   `testpackage` — white-box test files renamed to
   `*_internal_test.go`); three `//nolint:tagliatelle` directives keep
   the snake_case JSON wire/disk formats unchanged; `make check` green
+- 2026-08-07 validate configuration on startup, fail fast on bad
+  config (closes #52): a config value that is set but unparseable or
+  invalid aborts startup naming the key and value (defaults apply only
+  to omitted keys), unknown config keys abort startup, a malformed
+  config file aborts instead of being skipped, and `state_dir` is
+  verified creatable and writable before the listener binds
 - 2026-08-07 manual test pass of the auth and encrypted URL flows
   against a locally built and running `pixad` (built from `main` at
   `6573b9d`, port 18099, local throwaway config); all six checks
@@ -83,7 +89,6 @@ fill up
 
 # Future Steps
 
-- P0: validate configuration on startup, fail fast on bad config
 - P1: implement blocked networks configuration to extend SSRF
   protection
 - P1: rate limit global concurrent upstream fetches to prevent