…al reconnect (nesquena#1367)

Follow-up to v0.50.249 / PR nesquena#1365 absorbing Opus SHOULD-FIX #2.
Originally reset out of nesquena#1365 because the reviewer flagged it as
out-of-scope; brought back per follow-up guidance that
correctness-improving changes should ship even when out of scope.

The clarify SSE health timer at static/messages.js:1715 was an
unconditional 60s force-reconnect, not the 'no event in 60s' detector
its comment claimed. Now actually a stale-detector that tracks
lastEventAt on initial+clarify event arrivals; only reconnects when
the gap exceeds 60s. Under healthy conditions the timer never fires.

Co-authored-by: nesquena-hermes <[email protected]>

Both flagged by pre-release Opus advisor; both clearly defensive and small
enough to absorb in-release per the reviewer-flagged-fix-in-release-not-followup
policy.

SHOULD-FIX #1 (api/routes.py:_clear_stale_stream_state, ~25 LOC):
After the metadata-only reload (nesquena#1559 Layer 2), the local 'session'
variable is reassigned to the full-load object but the caller still holds
the original metadata-only stub. /api/session then returns the stale
active_stream_id at routes.py:1791, causing the frontend to attempt one
ghost SSE reconnect before recovering. Fix: capture original_stub at
function entry, then patch its in-memory active_stream_id and pending_*
fields to None after both the early-return (full-load already cleared)
path AND the successful-mutation path. Now the caller's read returns
fresh state, no ghost reconnect.

SHOULD-FIX #2 (api/models.py:Session.save, ~20 LOC):
The .bak write at api/models.py:436 used write_text() which truncates-
then-writes — a crash mid-write or concurrent backup-producing save
could leave a torn .bak. Recovery defends correctly (JSONDecodeError →
returns -1 → 'no_action'), so the failure mode was 'backup lost' not
'spurious restore'. Fix: tmp + os.replace pattern matching the main file
write at line 446-453. Now backup either lands cleanly or doesn't land
at all.

4026/4026 tests pass post-absorb.

SHOULD-FIX #1 (renamed-root client cross-alias): drop strict-equality client
filter at static/sessions.js:1853. Server-side _profiles_match cross-aliases
'default'-tagged rows to a renamed root 'kinni'; the strict-equality client
would reject them, dropping every legacy session for renamed-root users. The
server is now solely authoritative for profile scoping.

SHOULD-FIX #2 (messaging-source dedupe ordering): _keep_latest_messaging_session_per_source
now runs AFTER the profile filter at api/routes.py:2078. Before, it ran on
the merged-cross-profile list with profile-blind keys, discarding the older
profile's row across profiles before the scope filter — leaving zero rows for
any messaging identity the active profile shared with another profile.

NIT #3: _projects_migrated flag now set only AFTER successful save_projects.
NIT #4: cleaned dead test code in test_is_root_profile_invalidation_drops_stale.
NIT #5: _create_profile_fallback's clone_from=='default' literal now routes
through _is_root_profile() for parity with the 5 other callsites.

+2 regression tests pin the SHOULD-FIX shapes:
- test_keep_latest_messaging_runs_after_profile_filter (source-string ordering)
- test_static_sessions_js_trusts_server_profile_scoping (no client re-filter)

4173 -> 4175 tests pass. 0 regressions.