feat(auth): initial implementation — authmw and rbac (v1.0.0)

Introduces code.nochebuena.dev/einherjar/auth — the provider-agnostic HTTP
authentication and authorization layer of the Einherjar framework. Absorbs
two micro-lib packages (httpauth, rbac) as sub-packages, replacing the
Identity-only context model with a SecurityBag-native design and adding a
composable enrichment chain.

authmw:
- BagEnricher function type — enriches the request-scoped SecurityBag after
  the base Identity is built; registered via WithBagEnricher; multiple
  enrichers run in order, each receiving the bag from the previous
- IdentityEnricher interface — application-layer contract for loading user
  data from uid+claims
- EnrichmentMiddleware — builds SecurityBag from uid+claims, runs enricher
  chain, stores via security.SetBagInContext; 401 on missing uid, 500 on
  enricher error; routes all errors through httputil.Error
- AuthzMiddleware — per-route permission gate; 401 on missing identity,
  403 on provider error (fail-closed) or insufficient permissions
- EnrichOpt type + WithTenantHeader (reads TenantID from header, implemented
  as a BagEnricher) + WithBagEnricher (registers custom enrichers for
  hardware IDs, grant codes, or any bag attribute)
- SetTokenData / GetClaims — integration contract for auth-jwt / auth-firebase

rbac:
- NewClaimsPermissionProvider — reads flat JWT claim bitmasks from context;
  wildcard "*" fallback; handles int64/float64/json.Number; zero DB calls
- NewCachedPermissionProvider — TTL cache wrapping any PermissionProvider;
  default key "rbac:{uid}:{resource}" or "rbac:{tenantID}:{uid}:{resource}";
  TenantID sourced from SecurityBag automatically; accepts ...CachedOpt
- CachedOpt type + WithCacheKey — overrides the key function for extra
  dimensions (hardware IDs, grant codes read from bag attributes)
- NewChainPermissionProvider — tries providers in order; first non-zero wins;
  errors short-circuit; typical pattern: claims → cached DB fallback
- Cache interface — pluggable backend satisfied by cache-valkey via duck typing

Compliance test (package auth_test) enforces CT-6 (≤1 exported TypeSpec/file),
compile-time interface satisfaction, and behavioural coverage across the full
middleware and provider surface: enrichment success/failure, tenant header,
custom BagEnricher, bag-in-context, authz allowed/denied/error, claims
hit/wildcard/missing/float64, cached hit/miss/error/tenant-key/custom-key,
chain first-non-zero/fallthrough/error.

Depends on contracts v1.0.0, core v1.0.0, web v1.0.0.

- identifiable.go: package-level Module variable (observability.Identifiable) for version
  identification — auth is middleware-only; not registered with the launcher

.gitea/pull_request_template.md

@@ -0,0 +1,70 @@
+## Summary
+
+<!-- One or two sentences: what does this PR do and why? -->
+
+---
+
+## Type of change
+
+- [ ] Bug fix — non-breaking change that resolves an issue
+- [ ] New feature — non-breaking addition of functionality
+- [ ] Breaking change — alters existing behavior or public API
+- [ ] Documentation update
+- [ ] Refactor — no functional change, no new API surface
+- [ ] Test improvement
+
+---
+
+## Description
+
+<!--
+Provide enough context for a reviewer who was not in the room:
+- What problem does this solve?
+- What approach did you choose, and why?
+- Were there alternatives you considered and rejected?
+- Any known limitations or follow-up work?
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+
+---
+
+## Testing
+
+- [ ] I added or updated tests that cover my changes
+- [ ] All tests pass locally — `go test ./...`
+- [ ] No formatting issues — `gofmt -l .` produces no output
+- [ ] No vet warnings — `go vet ./...` is clean
+
+---
+
+## Checklist
+
+- [ ] At most one exported type per non-test `.go` file (CT-6)
+- [ ] No new external dependencies added without prior discussion in an issue
+- [ ] Public API changes are reflected in `CHANGELOG.md`
+- [ ] Breaking changes include a migration note in the PR description above
+
+---
+
+## Contributor License Agreement
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