feat(media): Add image variant support - v1.4.0 Phase 2

Implement automatic generation of multiple image sizes for responsive
delivery and enhanced feed optimization.

Changes:
- Add migration 009 for media_variants table with CASCADE delete
- Define variant specs: thumb (150x150 crop), small (320px), medium (640px), large (1280px)
- Implement generate_variant() with center crop for thumbnails and aspect-preserving resize
- Implement generate_all_variants() with try/except cleanup, pass year/month/optimized_bytes explicitly
- Update save_media() to generate all variants after saving original
- Update get_note_media() to include variants dict (backwards compatible - only when variants exist)
- Record original as 'original' variant type

Technical details:
- Variants use explicit year/month parameters to avoid fragile path traversal
- Pass optimized_bytes to avoid redundant file I/O
- File cleanup on variant generation failure
- Skip generating variants larger than source image
- variants key only added to response when variants exist (pre-v1.4.0 compatibility)

All existing tests pass. Phase 2 complete.

Per design document: /home/phil/Projects/starpunk/docs/design/v1.4.0/media-implementation-design.md

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

migrations/009_add_media_variants.sql

@@ -0,0 +1,21 @@
+-- Migration 009: Add media variants support
+-- Version: 1.4.0 Phase 2
+-- Per ADR-059: Full Feed Media Standardization (Phase A)
+
+-- Media variants table for multiple image sizes
+-- Each uploaded image gets thumb, small, medium, large, and original variants
+CREATE TABLE IF NOT EXISTS media_variants (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    media_id INTEGER NOT NULL,
+    variant_type TEXT NOT NULL CHECK (variant_type IN ('thumb', 'small', 'medium', 'large', 'original')),
+    path TEXT NOT NULL,           -- Relative path: YYYY/MM/uuid_variant.ext
+    width INTEGER NOT NULL,
+    height INTEGER NOT NULL,
+    size_bytes INTEGER NOT NULL,
+    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    FOREIGN KEY (media_id) REFERENCES media(id) ON DELETE CASCADE,
+    UNIQUE(media_id, variant_type)
+);
+
+-- Index for efficient variant lookup by media ID
+CREATE INDEX IF NOT EXISTS idx_media_variants_media ON media_variants(media_id);