feat: add task metadata tags

[deepjoy]

Summary

Add a schema-free key-value metadata layer to tasks — enabling filtering, grouping, and display without deserializing task payloads. Tags are immutable after submission (identity metadata, not progress state).
Fixes #19

Changes

Data model

• tags: HashMap<String, String> on TaskSubmission, TaskRecord, TaskHistoryRecord
• .tag(key, value) builder on TaskSubmission, BatchSubmission::default_tag(), TypedTask::tags() trait method
• Migration 007_tags.sql: task_tags and task_history_tags tables with (key, value) indexes
• StoreError::InvalidTag variant with validation constants (MAX_TAG_KEY_LEN=64, MAX_TAG_VALUE_LEN=256, MAX_TAGS_PER_TASK=32)

Write path

• Tags inserted atomically with task submission (within existing BEGIN IMMEDIATE transaction)
• Supersede: old tags deleted, new tags inserted (handles both in-place UPDATE and DELETE+INSERT paths)
• All terminal transitions (complete, fail, cancel, expire, supersede, dependency_failed) copy tags to task_history_tags via insert_history()
• Recurring re-creation: tags read into memory before old task deletion, then re-inserted for the new instance
• Child inheritance: ParentContext struct on ChildSpawner merges parent tags under child tags (child wins on conflict)

Read path

• populate_tags() batch loader using SELECT ... WHERE task_id IN (...) — called after row mapping in all query methods that return full records (including pop_next/pop_by_id)
• Separate populate_history_tags() for history records

Query API (on TaskStore and Scheduler)

• tasks_by_tags(filters, status) — AND semantics via INNER JOIN per filter
• count_by_tags(filters, status) — count variant
• count_by_tag(key, status) — group by tag value, sorted by count desc
• tag_values(key) — distinct values with counts
• cancel_by_tag(key, value) — cancel all matching active tasks

Events

• tags field added to TaskEventHeader, populated from TaskRecord::event_header()