refactor(core): overhaul state transition architecture, persistent job queue, and API layer

- Implement atomic state transitions: add EntryService.PatchEntry() with EntryDiff computation and EventEntryTransition event emission.
- Add persistent SQLite task queue (JobEngine): add jobs table with crash recovery, exponential backoff retries, concurrency control, and idempotency key deduplication.
- Decompose monolithic HTTP handlers: split handlers.go into domain handlers (auth, feeds, entries, jobs, settings, rules, sse).
- Implement real-time Server-Sent Events (/api/events/stream) for multi-tab/device state synchronization.
- Fix SQLite concurrency: apply busy_timeout(10000) and journal_mode(WAL) via DSN query parameters to resolve SQLITE_BUSY (5).
- Fix AI settings & prompt loading: unmarshal JSON from ai_settings, apply configured prompt templates, and enforce strict enabled/API key validations.
- Fix undo share state: remove omitempty from ShareUUID to serialize null for state resets.
- Fix middle list counter: compute true unread/total stats from DB and clean switcher button labels.
- Adapt mobile UI for iOS: add viewport-fit=cover in index.html and calibrate safe-area insets for the bottom navigation and article reader toolbar.
- Add architectural documentation and worklog in docs/009_worklog.md.

internal/models/models.go

@@ -67,7 +67,7 @@ type FeedEntry struct {
 	PublishedAt time.Time               `json:"published_at"`
 	IsRead      bool                    `json:"is_read"`
 	IsStarred   bool                    `json:"is_starred"`
-	ShareUUID   *string                 `json:"share_uuid,omitempty"`
+	ShareUUID   *string                 `json:"share_uuid"`
 	Variants    map[string]EntryVariant `json:"variants,omitempty"`
 	Metadata    map[string]interface{}  `json:"metadata,omitempty"`
 	CreatedAt   time.Time               `json:"created_at"`
@@ -175,3 +175,96 @@ type ChangePasswordRequest struct {
 	OldPassword string `json:"old_password"`
 	NewPassword string `json:"new_password"`
 }
+
+// Job represents a background task in the persistent job queue
+type Job struct {
+	ID             string                 `json:"id"`
+	Type           string                 `json:"type"`
+	Payload        map[string]interface{} `json:"payload"`
+	Status         string                 `json:"status"` // 'pending', 'running', 'completed', 'failed', 'cancelled'
+	Progress       int                    `json:"progress"`
+	Result         map[string]interface{} `json:"result,omitempty"`
+	ErrorMessage   string                 `json:"error_message,omitempty"`
+	Attempts       int                    `json:"attempts"`
+	MaxAttempts    int                    `json:"max_attempts"`
+	IdempotencyKey string                 `json:"idempotency_key,omitempty"`
+	RunAt          time.Time              `json:"run_at"`
+	CreatedAt      time.Time              `json:"created_at"`
+	UpdatedAt      time.Time              `json:"updated_at"`
+}
+
+// JobFilter is used to filter job listings
+type JobFilter struct {
+	Type   string
+	Status string
+	Limit  int
+	Offset int
+}
+
+// PatchEntryRequest is the body for PATCH /api/entries/{id}
+type PatchEntryRequest struct {
+	IsRead    *bool                    `json:"is_read,omitempty"`
+	IsStarred *bool                    `json:"is_starred,omitempty"`
+	Metadata  map[string]interface{}   `json:"metadata,omitempty"`
+	Variants  map[string]*EntryVariant `json:"variants,omitempty"`
+}
+
+// EntryDiff represents the delta between OldState and NewState of an Entry
+type EntryDiff struct {
+	FieldsChanged   []string `json:"fields_changed"`   // e.g. ["is_read", "is_starred", "metadata.tags"]
+	VariantsAdded   []string `json:"variants_added"`   // e.g. ["readability_full_text"]
+	VariantsChanged []string `json:"variants_changed"` // e.g. ["ai_summary"]
+	VariantsRemoved []string `json:"variants_removed"`
+}
+
+// HasVariantAddition checks if a specific variant key was newly added in this transition
+func (d EntryDiff) HasVariantAddition(variantKey string) bool {
+	for _, k := range d.VariantsAdded {
+		if k == variantKey {
+			return true
+		}
+	}
+	return false
+}
+
+// HasVariantChange checks if a specific variant key was modified in this transition
+func (d EntryDiff) HasVariantChange(variantKey string) bool {
+	for _, k := range d.VariantsChanged {
+		if k == variantKey {
+			return true
+		}
+	}
+	return false
+}
+
+// HasFieldChange checks if a specific top-level or metadata field changed
+func (d EntryDiff) HasFieldChange(fieldName string) bool {
+	for _, f := range d.FieldsChanged {
+		if f == fieldName {
+			return true
+		}
+	}
+	return false
+}
+
+// PromptTemplate represents an AI prompt template
+type PromptTemplate struct {
+	ID          string `json:"id"`
+	Name        string `json:"name"`
+	Description string `json:"description,omitempty"`
+	Prompt      string `json:"prompt"`
+	IsDefault   bool   `json:"is_default,omitempty"`
+}
+
+// AISettings represents AI provider configuration
+type AISettings struct {
+	Enabled     bool             `json:"enabled"`
+	Provider    string           `json:"provider"`
+	BaseURL     string           `json:"base_url"`
+	APIKey      string           `json:"api_key"`
+	Model       string           `json:"model"`
+	Temperature float64          `json:"temperature"`
+	MaxTokens   int              `json:"max_tokens"`
+	Prompts     []PromptTemplate `json:"prompts"`
+}
+