Add conversation pkg

internal/db/schema/1.init.sql

@@ -53,15 +53,16 @@ CREATE TABLE IF NOT EXISTS conversations (
 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. 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).
+-- stores the wire-format JSON for the message — the exact JSON object that
+-- appears in the OpenAI Chat Completions messages array; a CHECK constraint
+-- keeps it valid JSON. The role 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 (
     conversation_id TEXT NOT NULL REFERENCES conversations(id) ON DELETE CASCADE,
-    content         TEXT NOT NULL,
+    content         TEXT NOT NULL CHECK (json_valid(content)),
     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)),