docs: add ADR-0002 for Media Manager backend contract and mock adapter

Documents the MediaAsset model, future GET/POST/DELETE/PATCH /media contract,
and the MediaRepository interface (Mock IndexedDB-backed now, Http later via
DI swap) that Sprint 4 will implement against. Media assets never enter the
Bootstrap model, consistent with ADR-0001.

docs/context/adrs/ADR-0002-media-manager-contract.md

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+---
+id: ADR-0002
+title: Media Manager backend contract and mock storage adapter
+status: active
+date: 2026-07-15
+tags: ["architecture", "media", "backend-gap", "repository-pattern"]
+---
+
+## Context
+
+Sprint 4 (Media Manager) needs a media library: upload, browse, delete, and pick
+images/files for use across Product Editor, Static Pages (CMS), and Branding.
+No media backend exists yet — `/media` currently routes to a "coming soon"
+placeholder (`BackofficeComingSoonPageComponent`), and `docs/BACKEND.md` does
+not document any upload/storage endpoint. This mirrors the already-documented
+draft-publish-flow gap in the Project Editor (see `PE-20260713T010000Z-0003`):
+build the real contract, then implement a client-side mock adapter behind the
+same interface so the UI never needs to change when the backend ships.
+
+## Decision
+
+- **Domain model** `MediaAsset`: `{ id, url, thumbnailUrl?, filename, mimeType,
+  size, width?, height?, altText?: Record<locale, string>, tags?: string[],
+  createdAt }`. `altText` follows the platform's `translations.{lang}` rule
+  (ADR-0001) — never a flat string.
+- **Repository contract** (future backend, to be implemented server-side):
+  - `GET /media?page=&pageSize=&search=` → paginated `MediaAsset[]`
+  - `POST /media/upload` (multipart) → `MediaAsset`
+  - `DELETE /media/:id` → 204
+  - `PATCH /media/:id` (altText/tags only) → `MediaAsset`
+- **Frontend abstraction**: a `MediaRepository` interface (Repository pattern,
+  per `docs/context/features/*` conventions) with two implementations selected
+  via DI token:
+  - `MockMediaRepository` — stores assets in IndexedDB (not localStorage: binary
+    blobs need it) as an interim store until the backend exists. Data URLs are
+    generated for rendering; the shape returned matches `MediaAsset` exactly.
+  - `HttpMediaRepository` — thin wrapper over the endpoints above, added when
+    the backend ships. Swapping providers is the only change required.
+- **Media never enters the Bootstrap model.** Like products/orders/users, media
+  assets are runtime admin data, not tenant configuration — consistent with
+  ADR-0001's rule that Bootstrap contains only what's needed before the app
+  starts.
+- **Media Picker** is a standalone, reusable dialog (built on the existing
+  `app-dialog` Design System primitive) so Product Editor and CMS editors
+  consume the same selection UI instead of each building their own.
+
+## Consequences
+
+- Any feature needing to reference an image (product gallery, static page
+  hero, branding logo) does so via `MediaAsset.url`/`id`, obtained through the
+  shared Media Picker — never a raw file input duplicated per feature.
+- When the backend ships, only `MediaRepository`'s DI provider changes; no
+  component or facade code should need to change.

docs/context/features/media-manager/FACTS.jsonl

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+{"id":"MM-20260715T000000Z-0001","subject":"media-backend","predicate":"is","object":"not implemented yet; /media routes to BackofficeComingSoonPageComponent; GET /media, POST /media/upload, DELETE /media/:id, PATCH /media/:id are the documented backend gap","src":["docs/context/adrs/ADR-0002-media-manager-contract.md","src/app/app.routes.ts"],"status":"active","kind":"constraint","confidence":"high","updated_at":"2026-07-15T00:00:00Z","tags":["media-manager","backend-gap"]}
+{"id":"MM-20260715T000000Z-0002","subject":"media-storage","predicate":"is-implemented-by","object":"MediaRepository interface with MockMediaRepository (IndexedDB-backed, interim) and HttpMediaRepository (future) selected via DI token; media assets never enter the Bootstrap model","src":["docs/context/adrs/ADR-0002-media-manager-contract.md"],"status":"active","kind":"decision","confidence":"high","updated_at":"2026-07-15T00:00:00Z","tags":["media-manager","repository-pattern"]}