fix(sessions): use UPSERT in Save() to persist first-run cron sessions

[duhd-vnpay]

Fixes #379

Problem

PGSessionStore.Save() uses UPDATE sessions ... WHERE session_key = $19. When a cron job runs for the first time, the session row doesn't exist in DB yet. The UPDATE affects 0 rows and returns nil — silently losing the session.

This causes two cascading failures:

1. Cron sessions never persist — exist only in memory, lost on restart
2. Stale cache on retry — ExecuteWithRetry reads leftover messages from the failed attempt's in-memory cache, polluting retries with empty user messages + "..." assistant fallback. The stale messages cause LLM 400 errors (messages.0.content.0.text.text: Field required), and all retry attempts fail with the same error.

Fix

Change Save() from UPDATE to INSERT ... ON CONFLICT (tenant_id, session_key) DO UPDATE SET ....

Before:

UPDATE sessions SET messages = $1, ... WHERE session_key = $19 AND tenant_id = $20
-- 0 rows affected → silent data loss

After:

INSERT INTO sessions (id, session_key, tenant_id, messages, ...)
VALUES ($1, $2, $3, $4, ...)
ON CONFLICT (tenant_id, session_key) DO UPDATE SET
  messages = EXCLUDED.messages, ...
-- Always persists: INSERT on first run, UPDATE on subsequent saves

Why UPSERT over alternatives?

Option	Pro	Con
A: UPSERT (this PR)	Single atomic operation, no conditional logic	Generates new UUID on every save (ignored on conflict)
B: Check + INSERT	Two queries	Race condition between check and insert
C: Ensure row before agent loop	Requires caller discipline	Misses edge cases

The UUID generation cost on conflict is negligible — uuid.NewV7() is ~50ns.

Changes

File	Lines	Change
internal/store/pg/sessions_list.go	+32, -9	UPDATE → INSERT ... ON CONFLICT ... DO UPDATE

Verification

go build ./...                   # ✅ PG build
go build -tags sqliteonly ./...  # ✅ SQLite build
go vet ./...                     # ✅ Clean

The fix uses the same ON CONFLICT (tenant_id, session_key) unique index already used by GetOrCreate() (line 146 of sessions.go), confirming the constraint exists.

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