savedsearch: add /api/v1/saved-searches CRUD with 0002 migration

stats: add /api/v1/stats aggregate endpoint

Implements Phase 2 of lethe-web-ui-aggregates: a single-round-trip
/api/v1/stats endpoint that bundles per-tool rollup, daily time-series,
84-cell activity heatmap, top-cwd ranking, hour-of-day distribution, and
host split. All six aggregations run fresh per request; no server cache.

- internal/domain/stats/buckets.go: pure helpers DailyWindow, FillDaily,
  HeatmapWindow, HourWindow — fully deterministic, tested independently.
- internal/domain/stats/repository.go: Filter/Stats types and Repository.Stats
  — six SQL queries joined via sessions+turns, owner scope via OwnerScope
  (AllOwners / SpecificOwner / default), sparkline capped at 60 buckets.
- internal/domain/stats/handler.go: Handler.List — ?range=7d|30d|90d|all
  (default 30d); ?range=<other> → 400 INVALID; ?owner= non-admin → 403.
- internal/server/server.go: register *stats.Handler alongside project/sessions.
- cmd/lethe/main.go: add statsRepo/statsHnd to var block and steward graph.
- cmd/lethe/main_e2e_test.go: add stats.Repository/Handler to e2e graph.

project: add /api/v1/projects aggregation; sessions: add ?cwd= filter

- session.ListFilter gains Cwd *string; List appends working_dir = ?
  clause between Host and Since in fixed order (parameterised, column
  name is a literal)
- session.Handler reads ?cwd= and threads it to filter.Cwd; empty
  string treated as absent, consistent with other optional filters
- new package internal/domain/project:
  - Repository.List groups sessions by non-NULL working_dir in a single
    SQL round-trip; correlated subquery picks top_tool with tie-broken
    by tool ASC; Hosts/Tools deduped and sorted in Go after
    GROUP_CONCAT
  - Handler mounts GET /projects; resolveScope/clampLimit/clampOffset
    duplicated from session handler; ?owner= admin gating identical
- server.Server gains Projects *project.Handler inject field; mounted
  in /api/v1 Route block
- main.go and e2e test register projectRepo + projectHnd with steward

server: embed web SPA at /, wire build pipeline

- Add internal/server/web/embed.go with //go:embed all:dist and a
  SPA fallback shim: file-not-found → serve index.html at 200.
- Commit dist/.gitkeep and dist/index.html (placeholder) so go build
  works on a fresh clone; real build output stays gitignored.
- Mount web.Handler() as GET /* catch-all in server.go after /api/v1
  so API routes and probe endpoints shadow the wildcard.
- Add three server tests: ServesSPAAtRoot, SPAFallbackForNonAPIPath,
  APIPathsBypassSPA; update NotFoundReturnsProblemJSON for SPA era.
- Extend Justfile with web-{install,dev,build,test,lint,clean} targets;
  build now depends on web-build.
- Add node:20-alpine web-builder stage to Dockerfile; COPY dist into
  the Go builder stage before compiling.

feat(cmd): wire server with /healthz /readyz /metrics + authed /api/v1

Phase 9 of lethe-server: thin main.go that loads config, registers every
steward asset, and orchestrates Inject -> Init -> Start -> wait -> Stop ->
Destroy. Compensates for the Phase 4 finding (steward.Manager does not
unwind on Init failure) by tracking destroyer-implementing assets in a
parallel slice and invoking Destroy in reverse registration order with a
per-call timeout when Init or Start fails.

Server.Start now opens its TCP listener synchronously and exposes the
bound address via Addr(), so the e2e smoke can bind to 127.0.0.1:0 and
discover the kernel-assigned port.

Adds an end-to-end smoke test that drives the real steward graph
(in-memory SQLite, real loopback listener, forward-auth) through ingest +
sessions list/detail for two users with the same composite session key,
proving owner isolation reaches all the way through the trust boundary.

Deletes internal/platform/health/steward_unwind_test.go: the canary's
purpose was to surface the unwind gap so Phase 9 could compensate, which
it now does.

README updated with consolidated curl quickstart (forward-auth + OIDC
bearer variants), trust-chain diagram and the proxy-must-strip-Remote-*
spoofing note, response-shape documentation for the API surface, and an
operational notes section covering health, metrics, lifecycle and logs.

feat(http): chi server with middleware stack + RFC 7807 problem renderer