Add multi-user features: i18n, payments, multi-model AI, focus areas

Replace single report type with composable analysis: depth levels
(basic/standard/full) + focus area multi-select (psychology, business,
marketing, content, audience, sentiment). Multi-step inline keyboard
flow guides users through selection.

- i18n: English + Russian, auto-detect from Telegram, /lang override
- AI providers: Anthropic + OpenRouter via AIClient abstraction
- Telegram Stars payments with per-depth pricing and free trial
- SQLite (aiosqlite) for users, analyses, payments tracking
- User middleware for auto-registration and language detection
- Report persistence: save .md locally, offer file download
- New commands: /features, /prices, /lang
- Composable prompt system: depth modifiers + focus area fragments

bot/prompts/depth.py

@@ -0,0 +1,19 @@
+from bot.models import Depth
+
+DEPTH_CHUNK_MODIFIERS: dict[Depth, str] = {
+    Depth.BASIC: "Provide a brief, high-level summary. Focus on the most important points only. Be concise.",
+    Depth.STANDARD: "Provide a structured summary with moderate detail. Cover key patterns and notable examples.",
+    Depth.FULL: (
+        "Provide an exhaustive, detailed analysis. Include specific examples, quotes, "
+        "numerical data, and subtle patterns. Leave nothing significant out."
+    ),
+}
+
+DEPTH_SYNTHESIS_MODIFIERS: dict[Depth, str] = {
+    Depth.BASIC: "Keep the report concise and actionable. Use short sections with bullet points.",
+    Depth.STANDARD: "Provide a well-structured report with moderate depth. Balance brevity and detail.",
+    Depth.FULL: (
+        "Produce a comprehensive, in-depth report. Include detailed analysis, specific evidence, "
+        "data-backed observations, and strategic recommendations. Be thorough."
+    ),
+}