Phase F: NetworkMonitor, offline banner, README & architecture docs

- NetworkMonitor: ConnectivityState Flow using ConnectivityManager.NetworkCallback
- ListsScreen & ViewModel: live offline indicator banner when disconnected
- Documentation:
  - docs/ARCHITECTURE.md: system design & tech stack overview
  - docs/SYNC.md: HLC timestamping, op_log outbox & LWW projection specification
  - docs/API.md: REST API endpoint specification
  - README.md: quickstart guide for backend, docker compose & Android app
- Verification: backend & android test suites 100% green ✅

docs/SYNC.md

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+# Mitbringsl – Synchronization Protocol & Conflict Resolution
+
+## 1. Synchronization Model
+
+Mitbringsl uses an **Append-Only Operation Log (`op_log`)** with **Hybrid Logical Clock (HLC)** timestamps and **Last-Write-Wins (LWW)** projections.
+
+---
+
+## 2. Hybrid Logical Clock (HLC)
+
+Every operation carries a 64-bit integer timestamp `hlc_ts`:
+
+$$\text{hlc\_ts} = (\text{wall\_ms} \ll 16) \mid \text{counter}$$
+
+- **`wall_ms`** (48 bits): Unix epoch milliseconds.
+- **`counter`** (16 bits): Logical counter disambiguating events produced within the same millisecond.
+
+### Guarantees
+1. **Strict Monotonicity**: $t_{n+1} > t_n$ for all events generated on the same device.
+2. **Causal Ordering**: If event $B$ was generated after observing event $A$, then $HLC(B) > HLC(A)$.
+
+---
+
+## 3. Operations (`op_log`)
+
+All state modifications are represented as structured operations:
+
+| `op_type` | Target | Description | Payload Example |
+|---|---|---|---|
+| `list_create` | List UUID | Create list | `{"name":"Wocheneinkauf"}` |
+| `list_rename` | List UUID | Rename list | `{"name":"Supermarkt"}` |
+| `list_delete` | List UUID | Delete list | `{}` |
+| `item_add` | Item UUID | Add item | `{"name":"Milch","quantity":"2L"}` |
+| `item_update` | Item UUID | Update item | `{"name":"Hafermilch","checked":true}` |
+| `item_remove` | Item UUID | Delete item | `{}` |
+
+---
+
+## 4. Conflict Resolution (LWW & Tombstones)
+
+1. **Projection Updates**:
+   Database tables (`items`, `lists`) are projections of `op_log`. When an op arrives:
+   ```sql
+   UPDATE items
+   SET name = $1, hlc_ts = $2, updated_at = now()
+   WHERE id = $3 AND hlc_ts < $2 AND deleted_at IS NULL;
+   ```
+2. **Tombstones (`deleted_at`)**:
+   When an item or list is deleted (`item_remove`, `list_delete`), a tombstone is set (`deleted_at = now()`). Late-arriving offline `item_update` operations with smaller `hlc_ts` values will **never** revive a tombstoned entity.
+
+---
+
+## 5. Idempotent Push & Cursor Pull
+
+- **Idempotent Push (`POST /api/lists/{id}/ops`)**:
+  Each operation carries a `(client_id, client_seq)` tuple enforced by a `UNIQUE` constraint in Postgres (`op_log`). Re-sent requests return previous `(seq, hlc_ts)` assignments without duplicating side effects.
+- **Cursor Pull (`GET /api/lists/{id}/ops?since={seq}`)**:
+  Clients track `max(server_seq)` locally. Incremental sync fetches ops where `seq > cursor`, sorted by monotonic server sequence `seq ASC`.