feat: Add V2EX and HuggingFace as free sources (no API keys required)

[Alan5168]

Resolves #149

Summary

This PR adds two new free sources to last30days that require no API keys:

• V2EX: Chinese tech community hot topics (similar to Hacker News for Chinese developers)
• HuggingFace: Trending AI models with downloads, likes, and tags

Motivation

See detailed rationale in Issue #149:

Key Points:

1. V2EX fills Chinese tech community gap — Current sources are English-centric (Reddit, HN, X). V2EX covers Chinese developer discussions.
2. HuggingFace provides AI model trend data — Downloads and likes are real usage signals, not just discussion.
3. Zero-config, free-first — Both work immediately with no API keys, extending v2.9.6's philosophy.

Changes

File	Description
scripts/lib/v2ex.py	V2EX hot topics fetcher (public API)
scripts/lib/huggingface.py	HuggingFace models fetcher (public API)
scripts/last30days_lite.py	Standalone free-mode aggregator
scripts/last30days.py	Integration into main search pipeline
docs/free-mode.md	Documentation for new sources

Integration

Both sources are integrated into the main search pipeline:

• Added to VALID_SEARCH_SOURCES: v2ex, huggingface, hf
• Threaded search functions: _search_v2ex(), _search_huggingface()
• Normalized output format compatible with synthesis engine
• Relevance scoring: rank + engagement + topic overlap

Usage

# Free mode standalone
python3 scripts/last30days_lite.py --topic "AI coding"

# Integrated with full search (--sources flag)
python3 scripts/last30days.py --sources v2ex,huggingface,hn --topic "Llama 4"

Output Examples

V2EX:

## V2EX (10 条)
1. [尴尬了，可能招聘了一个不会手写代码的 AI 工程师]
   📍 career | 💬 100 | ⭐ relevance: 0.85

HuggingFace:

## HuggingFace (10 条)
1. [meta-llama/Llama-4-Maverick-17B-128E-Instruct]
   📥 1.2M downloads | ❤️ 3.5K likes | tags: llama, instruct

Testing

Both modules tested locally:

• ✅ V2EX API returns 10-30 hot topics successfully
• ✅ HuggingFace API returns trending models with engagement data
• ✅ Topic filtering works (case-insensitive substring match)
• ✅ Relevance scoring integrates with existing pipeline
• ✅ No API keys required — works immediately

Why This PR is Unique (vs other source requests)

PR/Issue	Source	Coverage	Auth Required	Unique Value
#134	GitHub	Repos, Issues	Optional GITHUB_TOKEN	Developer workflow
#136	GitHub	Same as #134	Optional	Duplicate of #134
#140	Tavily	Web search	TAVILY_API_KEY	Paid service
#149 (this)	V2EX + HF	Chinese tech + AI models	None	Free + unique coverage

Key differentiators:

• First Chinese-language source (V2EX)
• First AI-model-specific source (HuggingFace)
• Both 100% free — no config step needed

Comparison Table

Feature	Free Mode	Full Mode
Hacker News	✅	✅
Reddit	✅ RSS	✅ ScrapeCreators
V2EX	✅ NEW	❌
HuggingFace	✅ NEW	❌
X/Twitter	❌	✅
YouTube	❌	✅
TikTok/Instagram	❌	✅
Polymarket	❌	✅

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Expanding last30days coverage to global tech communities and AI model trends — zero API keys required.

Related: #134 (GitHub source), #128 (MiniMax API backend)