feat(phase8 #80 D8.6 chunk-3): drop agent_timeline_event table + DB persist (Redis-only wire emit)

[earayu]

Summary

Task #80 D8.6 hard-cut chunk-3 (final chunk) per PM scope lock: drop agent_timeline_event table + DB persist. Wire-emitter envelopes become ephemeral wire telemetry pushed only to the Redis live cache (live SSE + reconnect-within-TTL). Reload outside the Redis TTL has no historical events to replay; agent_message (D8.2 #74) is the single at-rest authority — already wired by chunk-2.

Per PM lock: NO UIMessage replay synthesis, NO new compensation for state.changed / tool.* lifecycle loss outside TTL. The 18 emit() call-sites in runtime.py keep their current live behavior.

What's removed

• EventService.to_event_envelope (only used to rehydrate DB rows).
• EventService.append_event no longer writes to db_ops.create_agent_timeline_event — envelope is composed locally with "evt" + 21-hex id and pushed only to Redis.
• EventService.get_events_after no longer falls back to db_ops.query_agent_timeline_events — Redis is the only source of replay events.
• AgentTimelineEvent SQLAlchemy model + agent_timeline_event table.
• db_ops.create_agent_timeline_event / query_agent_timeline_events.

What's kept

• EventService.adapt_event_envelope — canonical surface for tagging a wire envelope with user_activity (used by both append_event and get_events_after paths).
• redis_store.append_event / get_events_after / merge_runtime_state — wire emit + live cache + cursor tracking unchanged.
• All 18 runtime.py:emit() call-sites — turn lifecycle + tool lifecycle + state.changed + text.delta unchanged in behavior (only the DB write is removed under the hood).

Migration

New alembic head e8a1f5b2d4c7 (revises d8e6c2b4f1a9):

def upgrade():
    drop_index("ix_agent_timeline_event_type", "agent_timeline_event")
    drop_index("ix_agent_timeline_event_turn_id", "agent_timeline_event")
    drop_index("ix_agent_timeline_event_timestamp", "agent_timeline_event")
    drop_index("idx_agent_timeline_event_turn_timestamp", "agent_timeline_event")
    drop_table("agent_timeline_event")

Downgrade reconstructs both for rollback symmetry.

Caller sweep

Full grep for AgentTimelineEvent / create_agent_timeline_event / query_agent_timeline_events / to_event_envelope: zero remaining Python call-sites. Remaining hits are docstring narratives describing the removal.

PM 3 review-face gates

1. 18 emit() call-sites preserve live SSE behavior

• The only path removed under append_event is db_ops.create_agent_timeline_event(...).
• redis_store.append_event(envelope) + merge_runtime_state(turn_id, {"timeline_cursor": sequence}) retained.
• Envelope shape unchanged (AgentTimelineEventEnvelope with event_id / turn_id / sequence / timestamp / type / technical_type / label / status / actor / data / user_activity).

2. agent_timeline_event clean-deleted

• Model: gone from db/models.py.
• db_ops: 2 methods removed.
• Import: from aperag.domains.agent_runtime.db.models import AgentTimelineEvent dropped.
• Table: alembic migration drops table + 4 indices.
• DB fallback in get_events_after: gone.

3. No UIMessage replay synthesis introduced

• Zero new code paths that read UIMessage parts to fabricate timeline events.
• EventService surface stays exactly: append_event (Redis-only) + get_events_after (Redis-only) + adapt_event_envelope (pure helper).

Tests

• test_agent_runtime_v3.py: dropped query_agent_timeline_events mock; rewrote test_event_service_* to exercise adapt_event_envelope directly; renamed _build_event → _build_envelope.
• Full unit suite: 989 passed, 29 skipped, ruff + format clean.

D8.6 cleanup arc complete

After this PR merges:

• agent_artifact (chunk-2) + agent_timeline_event (chunk-3): both gone.
• agent_message is the single at-rest authority for assistant turn contents.
• Runtime's UIMessageStore.write at end-of-turn is the single write path.
• Wire-emitter events are ephemeral (Redis live cache only).

Test plan

• uv run pytest tests/unit_test/ -q — 989 passed
• uvx ruff check + format — clean
• Caller sweep for legacy timeline_event symbols → zero Python call-sites
• Migration upgrade/downgrade symmetry verified