feat(graph): GraphQL-native webhook surface (Phase 5a)

The webhook types, mutations, and resolvers, adapted from the pages.sr.ht
core-go template for spec's single-owner model.

- SDL: WebhookEvent (PROPOSAL_OPENED/MERGED/REJECTED), WebhookSubscription
  interface + UserWebhookSubscription, WebhookDelivery, WebhookPayload
  interface + ProposalEvent (carries a Proposal), cursor wrappers,
  `webhook` payload root field, and a `type Mutation` with
  createUserWebhook / deleteUserWebhook. No OAuth `client` field and no
  @access/@private directives — spec has no OAuth clients or scopes, so
  the owner gate is the entire ACL.
- Models: hand-written database.Model impls (UserWebhookSubscription,
  WebhookDelivery) so gqlgen autobinds rather than generates them; events
  via pq.Array; cursor keyset pagination.
- Resolvers: all owner-gated via authn (spec's ACL), using core-go's
  webhook engine — Validate, NewAuthConfig (INTERNAL, via the coreauth
  bridge), FilterWebhooks, WebhookContext.Exec for the sample, and the
  `webhook`→Payload(ctx) root. Proposal writes deliberately stay off this
  surface (only webhook mutations; the schema test now asserts exactly
  that).
- gqlgen.yml binds Cursor to core-go's model.Cursor; generated code
  regenerated with the pinned gqlgen v0.17.36 (reproducible).

Compiles and vets clean; existing graph read tests still pass. Runtime
context wiring and event firing are the next slices.

chore: promote fernet-go and go-ini to direct dependencies

authn/ imports both directly to decrypt the unified-login cookie and read
the instance ini, so the toolchain reclassifies them. No version changed
and no module was added.

Deliberately not running go mod tidy yet: bleve, chi, brant and the MCP
SDK have no importer until phases 2 and 3, and tidy would drop them from
go.mod, reintroducing it into every later parallel wave's file set.

feat: foundation — go.mod with every dependency, and the core/ domain

Phase 1 foundation commit. Two things, so that later parallel waves write
disjoint directories and never touch go.mod:

  - go.mod / go.sum carrying every external dependency the whole module will
    need (go-git, bleve, goldmark, chi, lib/pq, yaml.v3, the MCP SDK, brant,
    auxilia, testify, and the sr-ht-core fork). Populated by building a
    throwaway blank-import file, which is then deleted; `go mod tidy` runs
    once, at the very end of the build-out.
  - core/, the pure domain: owner/space names, safe relative paths, the
    globally-unique document ID grammar, frontmatter parsing and schema
    validation, `.spec.yml` policy with auto_merge glob matching, and the
    proposal state machine. Standard library plus yaml.v3, nothing else.

Two design invariants are enforced here rather than documented and hoped for:
"approved" is not a status (it is a property of the branch a document is
reachable from), and the proposal machine has exactly open/merged/rejected.
A per-space `.spec.yml` cannot reintroduce either.

Note on the sr-ht-core pin: the design calls for a `replace` onto
git.srht.bigb.es/~bigbes/core-go at c2c2f38, but that commit's go.mod still
declares `module git.sr.ht/~sircmpwn/core-go`, so Go rejects the replacement.
Both siblings pin the later dd418a20 under the canonical path with no
replace; this does the same.