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

session: extend List response with summary, turn_count, token totals, model

Add five aggregate fields to Session struct (Summary, TurnCount,
TokensInTotal, TokensOutTotal, Model) populated via a new
sessionListSelectColumns const that wraps correlated subqueries.
List uses the new const; Get is unchanged.

TestList_Aggregates covers: zero turns, long-content truncation to 200
chars, mixed-role model tracking (newest turn wins), and NULL token sums.

feat(session): list and detail JSON API with filters

feat(ingest): NDJSON ingest with chunked transactions and partial-accept

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