Add balance adjustments (M8)

- New balance_adjustments table with CRUD store, sync logging, and service methods
- SQL migrations restructured: embed fs.FS from internal/store/migrations/, apply in order via user_version
- WeekService.Balance combines closed-weeks delta + adjustments delta; BalanceSummary breakdown
- Four REST routes: GET/POST /api/balance/adjustments, PUT/DELETE /api/balance/adjustments/{id}
- Dexie schema v2 + sync apply cases for balance_adjustments
- API client: BalanceAdjustment type, balance namespace (list/create/update/delete)
- utils: composeDeltaMs / decomposeDeltaMs helpers + 8 new Vitest tests (19 total, all passing)
- History page: balance card breakdown line + full adjustments section with inline add/edit/delete

PLAN.md

@@ -88,13 +88,25 @@ CREATE TABLE settings_history (
 
 -- Sync event log (last-write-wins per entity row)
 CREATE TABLE sync_log (
-  entity        TEXT NOT NULL,              -- 'entries' | 'closed_days' | 'closed_weeks'
+  entity        TEXT NOT NULL,              -- 'entries' | 'closed_days' | 'closed_weeks' | 'balance_adjustments'
   entity_id     TEXT NOT NULL,
   op            TEXT NOT NULL,              -- 'upsert' | 'delete'
   version       INTEGER NOT NULL,           -- monotonic server-assigned
   payload       TEXT NOT NULL,              -- JSON snapshot
   PRIMARY KEY (entity, entity_id, version)
 );
+
+-- Balance adjustments: manual corrective entries that modify the overall overtime balance
+-- without touching week math. Signed delta_ms (positive = credit, negative = debit).
+CREATE TABLE balance_adjustments (
+  id           TEXT PRIMARY KEY,            -- client-generated UUIDv7 (sync-friendly)
+  delta_ms     INTEGER NOT NULL,            -- signed; non-zero enforced at service layer
+  note         TEXT NOT NULL DEFAULT '',    -- optional free-text reason
+  effective_at INTEGER NOT NULL,            -- unix ms; backdatable
+  created_at   INTEGER NOT NULL,
+  updated_at   INTEGER NOT NULL
+);
+CREATE INDEX idx_balance_adjustments_effective_at ON balance_adjustments(effective_at);
 ```
 
 ### API Surface (REST/JSON)
@@ -118,6 +130,13 @@ DELETE /api/days/{day_key}/close       (reopen day)
 GET    /api/weeks?from=YYYY-Www&to=YYYY-Www
 POST   /api/weeks/{week_key}/close                          -> ClosedWeek
 DELETE /api/weeks/{week_key}/close     (reopen week)
+GET    /api/weeks/balance                                   -> BalanceSummary
+
+# Balance adjustments
+GET    /api/balance/adjustments                             -> []BalanceAdjustment
+POST   /api/balance/adjustments        { delta_ms, note?, effective_at }  -> BalanceAdjustment
+PUT    /api/balance/adjustments/{id}   { delta_ms, note?, effective_at }  -> BalanceAdjustment
+DELETE /api/balance/adjustments/{id}
 
 # Settings
 GET    /api/settings                                        # current effective settings
@@ -243,7 +262,11 @@ Staged implementation:
 | 4 | Backend guard: reject future-day intervals | pending |
 | 5 | Bottom nav Today shortcut; delete /today route | pending |
 
-### M8 — Future
+### M8 — Balance adjustments ✅
+
+Manual corrective entries on the History page that adjust the overall overtime balance without touching week math. Separate `balance_adjustments` table with signed `delta_ms`, optional `note`, and `effective_at` (backdatable). Balance summary combines `Σ closed_weeks.delta_ms + Σ balance_adjustments.delta_ms`.
+
+### M9 — Future
 CSV/JSON export, monthly summary view.
 
 ## 7. Decisions & Rationale
@@ -260,5 +283,7 @@ CSV/JSON export, monthly summary view.
 | Week close settings | Use close-time date, not week Monday | Settings changed mid-week apply to that week's close |
 | weekDayKeys formula | `(weekday+6)%7` for days-since-Monday | Avoids sign bug when Jan 4 is Sunday (affects year 2026) |
 | Merged Today+Week | Single `/week` route with day selection | Reduces duplication; week context always visible |
-| Future intervals | Rejected by server (400) | Nonsensical to track time that hasn't happened |
+| Balance adjustments | Separate `balance_adjustments` table | Avoids conflating measured deltas with manual corrections; preserves week math invariant (`delta_ms = worked - expected`) |
+| Balance adjustment scope | Closed weeks only for auto balance | In-progress week delta shown separately in week view; mixing would make balance jitter |
+| Balance adjustment IDs | TEXT (UUIDv7, client-generated) | Consistent with `entries`; allows offline creation and sync |
 | Test framework | Vitest (frontend) + Go testing (backend) | Automated coverage for capability logic and key utilities |