synapse: a reaction continues its conversation, and the request becomes a record ...

Every Synapse event used to open its own service session, so two messages of
one Telegram chat landed in two unrelated chats. The dispatcher now returns a
`session_key` — derived from the payload according to the user's routing
document — and a later event carrying the same key joins the session that
opened it, within `reaction_session_ttl_minutes` of idle time (0 restores the
old always-fresh behaviour). A session that is still running is never joined:
`create_run` overwrites `state.run`, so a second run would orphan the first
one's subscribers — a busy thread gets a fresh session instead. When the
dispatcher names a different profile for the event, the session's profile is
switched through `set_profile`, never through `save()`, which does not rewrite
`name` or `profile_id`: the thread keeps its name and its history.

What the user reads in the transcript is now only the request. The event
record is a system message (`metadata.source == "synapse_event"`) drawn by a
new SynapseEventNotice.vue in the visual language of the recall badge, and the
framing plus the JSON envelope become a hidden user turn written once, on the
first event of a thread — not on every one. The instructions from Settings
reach the model through a new `current_reaction_instructions` ContextVar and
the `[Reaction session]` block of the system prompt, so they appear in no
transcript at all. The stored system record still has to reach the model, so
ContextBuilder's allowlist becomes `_LLM_SYSTEM_SOURCES = {task_note,
synapse_event}`.

The dispatcher gets a document of its own — how to route, rather than what to
do — stored beside the reaction instructions, versioned separately, and edited
in its own field in the settings panel. `synapse_instruction_versions` gains a
`doc` discriminator, so existing rows keep `'reaction'` and their history
verbatim. The new columns are added by `_MIGRATE`, and the index over `doc`
lives there too: `_DDL` runs first, so an index over a column the migration has
not added yet kills the whole batch on an existing database — which is what
the prod database is. A guard test keeps it that way.

Tests: 1776 passed, 1 skipped (backend), 242 passed (webclient, 28 files).

synapse settings: source_ready flag in GET/PUT response ...

Tells the UI whether Synapse-linked options are usable (source key
configured); not persisted, computed per request.

synapse reactions W3: reaction runner, notify tool, gateway hookup ...

- navi/synapse/reactions.py: fire-and-forget reaction run — per-user
  reactions_enabled gate, one-shot dispatcher meta-pass (visible profiles
  only, hidden dispatcher, secretary fallback), special=True session
  headed by the resolved profile, headless agent run via the orchestrator
  run registry, finalise: app push per completion_notify +
  low-level reaction.finished/failed event to Synapse
- navi/synapse/outbound.py: source-side emit (syn_* key from env,
  silent when not configured)
- navi/tools/notify.py: notify tool — message + level (info/warning/
  intervention), both legs per push_target, honest delivered/skipped result
- push/service.py: notify_custom without cooldown
- gateway: verified + non-duplicate delivery schedules the reaction
- profiles: notify added to native tools (7 profiles)

synapse reactions W1: special sessions flag + per-user reaction settings and instruction history

handbook alignment: profile.updated mirror, health contract, Synapse gateway ...

Handbook gaps closed (10-platform auth/health/notifications):

- webhooks: handle user.profile_updated — mirror the gnexus-auth profile
  into navi_users (name, contact fields, avatar_url — new column, boot
  migration); role/permissions keep their dedicated events
- auth: avatar_url now flows through the login upsert and the API-token
  resolution path
- /health: status is now the aggregate of the sub-checks (degraded embed
  or hive no longer reports plain ok); embed probe cached for 10 s; body
  grew the machine-readable checks{} map, legacy embed/hive payloads kept
- Synapse: s2s delivery gateway — POST /webhooks/synapse (both spellings),
  per-user delivery secrets (synapse_targets, Fernet-encrypted, shown
  once), verification via gnexus-synapse v0.1.2 client lib, idempotent
  event ids; deliveries are verified and logged for now — navi generates
  no outbound events by decision; web-push stays as the local browser
  channel
- webclient settings: Synapse deliveries panel (add/revoke targets)