feat(proto): add SetReadOnly + GetReadOnlyStatus RPCs

proto/database.proto

@@ -82,6 +82,10 @@ service DatabaseService {
     // Health and stats
     rpc HealthCheck(HealthCheckRequest) returns (HealthCheckResponse);
     rpc GetStats(GetStatsRequest) returns (GetStatsResponse);
+
+    // Read-only runtime state control
+    rpc SetReadOnly(SetReadOnlyRequest) returns (SetReadOnlyResponse);
+    rpc GetReadOnlyStatus(GetReadOnlyStatusRequest) returns (GetReadOnlyStatusResponse);
 }
 
 // ===== Replication Service =====
@@ -786,3 +790,29 @@ message MigrateCollectionTimestampsResponse {
     uint64 rows_migrated = 3;    // how many documents had their timestamps updated
     uint64 rows_skipped = 4;     // already in target range (idempotent resume)
 }
+
+// ===== Read-Only Control =====
+
+message SetReadOnlyRequest {
+    bool read_only = 1;
+}
+
+message SetReadOnlyResponse {
+    bool success = 1;
+    bool was_read_only = 2;     // previous state
+    string error = 3;
+}
+
+message GetReadOnlyStatusRequest {}
+
+message GetReadOnlyStatusResponse {
+    bool read_only = 1;
+    string reason = 2;
+    // Recovery outcome details (populated when auto-readonly triggered)
+    string recovery_outcome = 3;    // "trivial_success" | "fresh_install" | "snapshot_fell_back" | "wal_only_replay" | "forced_empty" | "failed"
+    string expected_snapshot = 4;
+    string snapshot_used = 5;
+    string failure_reason = 6;
+    uint64 wal_entries_replayed = 7;
+    uint32 snapshots_attempted = 8;
+}