Add db message metadata

internal/db/schema/1.init.sql

@@ -55,7 +55,8 @@ CREATE INDEX IF NOT EXISTS idx_conv_user ON conversations(user_id);
 -- Messages are ordered entries within a conversation. The content column
 -- stores the wire-format JSON for the message — the exact JSON object
 -- that appears in the OpenAI Chat Completions messages array. The role
--- column is extracted for querying without JSON parsing. The sequence
+-- column is extracted for querying without JSON parsing. Nullable metadata
+-- stores inference JSONB separately from model-visible content. The sequence
 -- column defines ordering within the conversation. Deleting a message
 -- cascades to nothing further (all message data is in the row).
 CREATE TABLE IF NOT EXISTS messages (
@@ -63,6 +64,7 @@ CREATE TABLE IF NOT EXISTS messages (
     content         TEXT NOT NULL,
     created_at      TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
     id              TEXT PRIMARY KEY,
+    metadata        BLOB CHECK (metadata IS NULL OR json_valid(metadata, 8)),
     role            TEXT NOT NULL,
     sequence        INTEGER NOT NULL,
     updated_at      TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
@@ -149,6 +151,8 @@ CREATE TABLE IF NOT EXISTS executions (
 
 CREATE INDEX IF NOT EXISTS idx_executions_message ON executions(message_id);
 CREATE INDEX IF NOT EXISTS idx_executions_pending ON executions(status) WHERE status = 'pending';
+-- Prevent duplicate execution rows for the same tool call in a message.
+CREATE UNIQUE INDEX IF NOT EXISTS idx_executions_call ON executions(message_id, tool_call_id);
 
 -- Default user. Temporary workaround until authentication is built.
 INSERT INTO users (id, name, password, role)