v1.13.0: message_parts table + dual-write at every tool_calls/tool_results site

Adds a granular message_parts table (one row per text/tool_call/tool_result
chunk) without changing any read path. Old messages.content / tool_calls /
tool_results columns remain authoritative for v1.13.0; this dispatch is
write-only mirroring so the AI SDK migration in v1.13.1 can flip read
authority without a backfill window.

Schema:
  CREATE TABLE message_parts (id, message_id FK ON DELETE CASCADE,
    sequence int, kind text CHECK (text|tool_call|tool_result|reasoning|step_start),
    payload jsonb, created_at, UNIQUE (message_id, sequence))

New module services/inference/parts.ts with two pure derive helpers
(partsFromAssistantMessage, partsFromToolMessage) and insertParts that
fan-outs a multi-row INSERT via postgres-js.

Wired dual-write at every site that writes tool_calls or tool_results:
- tool-phase.ts: assistant finalize UPDATE, executed-tool UPDATE,
  ask_user_input sentinel UPDATE
- messages.ts answer flow: DELETE pending tool_result part + INSERT
  answered one inside the existing sql.begin
- skills.ts: synthetic assistant + tool INSERTs both inside existing tx
- chats.ts fork: CTE clones parts via ROW_NUMBER pairing (source→dest
  message id mapping in one statement, no N+1)
- error-handler.ts finalizeCompletion: text part for plain text-only
  assistant turns

Deviation: tool-phase.ts finalize UPDATEs and finalizeCompletion text-part
write are not wrapped in fresh sql.begin transactions. Safe in v1.13.0
because JSON columns are authoritative for reads. v1.13.1 must wrap these
sites before flipping read authority — TODO comments added at each
unwrapped site referencing v1.13.1.

Tests: 8 new unit tests for the derive helpers in
services/__tests__/parts.test.ts. Existing 162 tests untouched. 170 total.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/server/src/routes/chats.ts

@@ -313,6 +313,28 @@ export function registerChatRoutes(
             AND created_at <= ${target.created_at}::timestamptz
             AND status = 'complete'
         `;
+        // v1.13.0: clone message_parts for the forked messages. Source and
+        // destination preserve ordering (the INSERT above orders by created_at,
+        // id) so a ROW_NUMBER pairing maps source.id → dest.id deterministically.
+        await tx`
+          WITH src AS (
+            SELECT id, ROW_NUMBER() OVER (ORDER BY created_at ASC, id ASC) AS rn
+            FROM messages
+            WHERE chat_id = ${source.id}
+              AND created_at <= ${target.created_at}::timestamptz
+              AND status = 'complete'
+          ),
+          dst AS (
+            SELECT id, ROW_NUMBER() OVER (ORDER BY created_at ASC, id ASC) AS rn
+            FROM messages
+            WHERE chat_id = ${chat!.id}
+          )
+          INSERT INTO message_parts (message_id, sequence, kind, payload)
+          SELECT dst.id, p.sequence, p.kind, p.payload
+          FROM message_parts p
+          JOIN src ON p.message_id = src.id
+          JOIN dst ON dst.rn = src.rn
+        `;
         return chat!;
       });