Web app: support multiple bank connections (multi-source)

[corentin-core]

Context

The app is deliberately single-connection: one Enable Banking consent, one API source, and a flat sync_runs log with no source dimension. This matches the epic #293 decision ("single shared budget, single consent"). Supporting several bank connections at once (e.g. BNP + Boursorama, each with its own consent and expiry) would be a natural extension once the need arises.

Captured while building the in-app alerting (#306) so the design intent isn't lost.

Proposed solution

An extension, not a rewrite — the domain/services layer already syncs per-source under the hood (BankSyncService takes a source + account name). What multi-source touches:

Zone	Today	Multi-source
Consent storage	one consent.json	N consents keyed by bank/account, each with its own expiry
perform_sync	syncs "the" consented account	loops over connections, one SyncRun per source
sync_runs table	no source column	add a source column; history filterable per bank
Consent banner	one status	aggregate (worst status, or a list)
Failed-sync banner + supersession	"the" consent	per source: correlate run.source with that source's consent created_at
Réglages	one connection panel	one card per bank (status + Sync now + history)

The #306 supersession rule (hide a failed-sync banner once superseded by a newer consent) generalizes cleanly to per-source — no rework.

Acceptance criteria

• Multiple Enable Banking consents can be stored and renewed independently.
• sync_runs records the source of each run; Réglages shows per-bank status and history.
• Consent and failed-sync banners reflect all connections, each supersession scoped to its own source.

Related issues

• Related to Web app for remote/mobile access (shared household budget) #293 (epic; single-consent was the initial scope)
• Follow-up from Web app: in-app alerting (sync_runs + Reglages + banner) #306 (in-app alerting)