feat(api): API keys + Redis rate limiting + usage stats

Add an optional API-key layer to the public read-only API. Keys grant
higher per-minute rate limits and attribute usage; anonymous callers are
still allowed at a lower IP-based budget.

- migration 0008_api_key: api_key table (sha256 hash only, plaintext shown once)
- apikey pkg: key generation + hashing
- ratelimit pkg: Redis fixed-window limiter + per-key usage counters; fails open
- public API middleware: X-API-Key / Bearer auth, X-RateLimit-* headers, 429+Retry-After
- admin: issue/list/revoke keys + usage view (API + UI tab)

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

api/internal/apikey/apikey_test.go

@@ -0,0 +1,60 @@
+package apikey
+
+import "testing"
+
+func TestGenerate(t *testing.T) {
+	key, hash, prefix, err := Generate()
+	if err != nil {
+		t.Fatalf("Generate: %v", err)
+	}
+	if !IsWellFormed(key) {
+		t.Fatalf("generated key not well-formed: %q", key)
+	}
+	if Hash(key) != hash {
+		t.Fatalf("Hash(key) != returned hash")
+	}
+	if len(prefix) != prefixLen || key[:prefixLen] != prefix {
+		t.Fatalf("prefix %q not a %d-char prefix of key %q", prefix, prefixLen, key)
+	}
+	if len(hash) != 64 {
+		t.Fatalf("hash not hex sha-256: %q", hash)
+	}
+}
+
+func TestGenerateUnique(t *testing.T) {
+	seen := map[string]bool{}
+	for i := 0; i < 100; i++ {
+		k, _, _, err := Generate()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if seen[k] {
+			t.Fatalf("duplicate key generated: %q", k)
+		}
+		seen[k] = true
+	}
+}
+
+func TestHashStableAndTrimmed(t *testing.T) {
+	if Hash("og_live_abc") != Hash("  og_live_abc  ") {
+		t.Fatal("Hash should ignore surrounding whitespace")
+	}
+	if Hash("a") == Hash("b") {
+		t.Fatal("distinct inputs must hash differently")
+	}
+}
+
+func TestIsWellFormed(t *testing.T) {
+	cases := map[string]bool{
+		"og_live_abcdefghijkl": true,  // body longer than 8 chars
+		"og_live_":             false, // empty body
+		"og_live_abc":          false, // body too short
+		"nope_abcdefghijkl":    false, // wrong prefix
+		"":                     false,
+	}
+	for in, want := range cases {
+		if got := IsWellFormed(in); got != want {
+			t.Errorf("IsWellFormed(%q) = %v, want %v", in, got, want)
+		}
+	}
+}