MushroomObserver / mushroom-observer / 27906510908

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Consolidate Source into ExternalLink relationship model (#4299 phase 1) (#4568)

* Consolidate Source into polymorphic ExternalLink model (#4299 phase 1)

Phase 1 (additive) of collapsing the Source table + Observation
source_id/external_id into the ExternalSite/ExternalLink M:N model. Additive
only — the phase-2 migration drops the old columns/table after the backfill
is verified in production.

ExternalLink is polymorphic (target_id/target_type) so it attaches to both
observations AND images — one model, one code path. This replaces the
separate per-image provenance mechanism: image import provenance (#4529)
becomes an import ExternalLink targeting the image.

Schema (additive migration):
- external_links: observation_id -> target_id + target_type; + external_id
  (canonical platform id for sync/dedup), relationship enum (default 0 =
  cross_reference), last_synced_at. At most one import (relationship=1) per
  target via a stored generated column + unique index. Composite dedup index
  (external_site_id, relationship, target_type, external_id) + a target index.
- external_sites: + description, last_successful_sync_at, url_template.
- images: unchanged (source_id/external_id stay until phase 2; no new column).

URL is now derived: ExternalSite#observation_url builds it from url_template
(+ external_id), falling back to base_url + id. external_links.url stays as a
nullable override for the rare site whose URL can't be templated; import links
store no url. (Per review: external_id is the canonical id; url has value only
as the freeform override.)

Models:
- ExternalLink: polymorphic target, relationship enum,
  only_one_import_per_target, link_url (override-or-derived), observation/=
  shims so the observation-only manual-link feature (controller, Query, API2,
  views) keeps working unchanged. user_id=0 future direction noted on the
  user association (deferred to image-provenance work, not #4299).
- ExternalSite: observation_url via template, inatural... (continued)

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