fix(CF-816): Self-healing session number sequence to prevent drift

session_sequences table fell behind when sessions were inserted with
explicit session_number (e.g., retro imports), causing duplicate key
violations on next auto-assigned number. Function now syncs forward
by checking MAX(session_number) before assigning.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

migrations/032_fix_session_sequence_drift.sql

@@ -0,0 +1,57 @@
+-- Migration 032: Fix session_sequences drift (CF-816)
+-- Problem: Retro-imported sessions with explicit session_number bypass the trigger,
+--          leaving session_sequences.next_number behind the actual MAX(session_number).
+--          Next auto-assigned number then collides with the unique index.
+-- Fix: Make get_next_session_number() self-healing by always checking actual max.
+
+-- Step 1: Replace the function with a self-healing version
+CREATE OR REPLACE FUNCTION get_next_session_number(p_project TEXT)
+RETURNS INTEGER AS $$
+DECLARE
+    v_seq_number INTEGER;
+    v_max_number INTEGER;
+    v_number INTEGER;
+BEGIN
+    -- Insert project if doesn't exist
+    INSERT INTO projects (key, name) VALUES (p_project, p_project)
+    ON CONFLICT (key) DO NOTHING;
+
+    -- Insert sequence if doesn't exist
+    INSERT INTO session_sequences (project, next_number)
+    VALUES (p_project, 1)
+    ON CONFLICT (project) DO NOTHING;
+
+    -- Get the actual max session_number for this project (handles external inserts)
+    SELECT COALESCE(MAX(session_number), 0) INTO v_max_number
+    FROM sessions
+    WHERE project = p_project;
+
+    -- Sync sequence forward if it fell behind (self-healing)
+    UPDATE session_sequences
+    SET next_number = GREATEST(next_number, v_max_number + 1),
+        last_updated = NOW()
+    WHERE project = p_project;
+
+    -- Now atomically increment and return
+    UPDATE session_sequences
+    SET next_number = next_number + 1,
+        last_updated = NOW()
+    WHERE project = p_project
+    RETURNING next_number - 1 INTO v_number;
+
+    RETURN v_number;
+END;
+$$ LANGUAGE plpgsql;
+
+-- Step 2: Sync all existing sequences to match actual data
+UPDATE session_sequences sq
+SET next_number = GREATEST(sq.next_number, sub.actual_max + 1),
+    last_updated = NOW()
+FROM (
+    SELECT project, COALESCE(MAX(session_number), 0) AS actual_max
+    FROM sessions
+    WHERE project IS NOT NULL
+    GROUP BY project
+) sub
+WHERE sq.project = sub.project
+  AND sq.next_number <= sub.actual_max;